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Effects of temperature and photoperiod on the duration of larval development in three species of Odonata. Procter, Dennis Lester Coor 1971

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THE EFFECTS OF TEMPERATURE AND PHOTOPERIOD ON THE DURATION OF LARVAL DEVELOPMENT IN THREE SPECIE OF ODONATA by DENNIS L. C. PROCTER B.Sc. (Hons). The University of Canterbury, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department "of ZOOLOGY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1971. In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t he L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s tudy . I f u r t h e r ag ree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d tha t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Co lumb ia V a n c o u v e r 8, Canada Date m-xk JLJdb^JJKr W7f TABLE OF CONTENTS Page ABSTRACT i« INTRODUCTION . 1 MATERIALS AMD METHODS 4 ORIGINS, DISTRIBUTIONS AND PATTERNS OF EMERGENCE . . 7 THE REARING EXPERIMENTS . . 11 The effect of temperature on the growth rate . 11 The effect of photoperiod on the growth rate . 15 The response of the f i n a l instar to the . . . . experimental conditions 1,7 The response of the eggs to the experimental conditions , 19 CONCLUSIONS 21 ACKNOWLEDGEMENT S . 20j. REFERENCES 25 TABLES Table Page 1 The patterns of emergence of E. boreale, L. K l a c i a l l s and L. quadramaculata 10 2 The growth rate data obtained-for E. boreale, L. p;laoialls and L. quadramaculata 12 3 The growth rate responses of L. p;lacialis and L. quadramaculata to temperature expressed as ratios relative to the growth rate of E. boreale. 14 4. The growth rate responses of L. pJLaolalls and L. quadramaculata to photoperlod expressed as ratios relative to the fastest growth rate shown by each species 14 Figure 1 Marion Lake: location of the s i x sites (numbered J.-6) from which the exuvia were collected . . . . 111 ABSTRACT Corbet has recognised two ecological types of Odonata from temperate regions. He has termed these "spring" and "summer" species, depending upon the presence of a diapause i n the f i n a l i n s t a r . Species possessing a diapause i n the f i n a l i n s t a r are by d e f i n i t i o n spring species. Corbet believes that spring and summer species represent stages In the colonisation of high l a t i t u d e s from the t r o p i c s . He considers that spring species represent the f i n a l stage i n adaptation to cold climates, becuase the diapause confers cold resistance on the f i n a l i n s t a r , and synchronises the short adult l i f e a f t e r the r e l a t i v e l y long l a r v a l period caused by the low temperatures. This study tested several hypotheses a r i s i n g from Corbet's scheme: (1) Corbet's assumption that spring species develop more slowly ( i . e . , have lower thermal c o e f f i c i e n t s for development) at high temperatures than summer species; (2) Corbet's assumption that spring species are more tolerant of low temperature than summer species, and (3) the hypothesis proposed i n t h i s study that spring species make the most general use of photoperiod i n regulating development. The nymphs of three species of Odonata, Enallagma boreale, Leucorrhinia p;lacialis and L i b e l l u l a quadramaculata, were reared under a number of combinations of temperature (10, 15, 20, 25 C) and photoperiod (6, 9, 12, 15, 18 h r ) . Enallapcma boreale. the summer species, developed more rapidly than the two spring species at every temperature. This 1v r e s u l t supports Corbet's hypothesis that summer species have higher thermal c o e f f i c i e n t s for growth t h a n do spring species, but does not support h i s hypothesis that spring species are more tolerant of low temperatures. Photoperiod s i g n i f i c a n t l y affected the rate of development 3-n Leucorrhinia g l a c i a l Is and L l b e l l u l a quadramaculata at 10 C and 15 G> but £nallap;ma boreale was not affected by photoperiod at any temperature. This r e s u l t supports the hypothesis that species from high l a t i t u d e s (spring species) are more l i k e l y to u t i l i s e photoperiod i n regulating development. Preliminary r e s u l t s suggest that Leucorrhinia g l a c i a l i s i s capable of continuously variable growth rate response to photoperiod. This i s the f i r s t time that t h i s response has been recorded i n an arthropod. v INTRODUCTION The time taken to complete l a r v a l development varies considerably between species of Odonata. The most rapid development has been observed In tropical species, such as members of the Family Lestidae, which may complete l a r v a l development within a few months (Corbet, 1963). On the other hand, temperate members of the Families Cordulegasteridae and Gomphldae probably require four years for development, and a few other species may take even longer (Corbet, loc. c i t . ) . Although the environmental temperature clearly affects the rate of growth, i t i s not the sole determinant. Calvert (1929), and Hodgkln and Watson (1958) have shown that species have different thermal coefficients for growth, that Is, d i f f e r In their capacity to respond to increasing temperature. The lat t e r authors showed that species inhabiting temporary pools had higher thermal coefficients for growth than slow-growing species from upland streams. In t r o p i c a l species, the thermal coefficient for growth and the environmental temperature probably determine the l a r v a l growth rate (Corbet, 1963). In temperate species, however, the duration of la r v a l development i s affected by additional factors, involving variable responses to both temperature and photoperiod (Corbet, loc. c i t . ; Lutz and Jenner, 1964; Lutz, 1968). L i t t l e i s known of how these additional factors operate, although one of their functions appears to be to synchronise emergence of the adult at the end of the long l a r v a l period (Corbet, loc. c i t . ) . - 2 -On the basis of their methods of synchronisation, Corbet (1954, 1956) has recognised two ecological types of dragonflies from temperate regions. He has termed these "spring" and "summer" species, depending upon the position of a diapause stage i n the l i f e history. Spring species by definition possess a diapause i n the f i n a l instar (Corbet, 1956, 1963). Because of the position of the diapause, spring species are also characterised by a short and re l a t i v e l y early emergence period. Summer species lack the f i n a l instar diapause. Consequently, the l a t t e r do not have the short and early emergence characteristic of the spring species. However, compared with tropical species, emergence i n summer species i s rel a t i v e l y synchronised. To explain t h i s , Corbet (1956) has suggested that an ascending series of lower temperature thresholds may regulate entry to the f i n a l instar, commencement of metamorphosis, and emergence. Corbet (loc. c i t . ) points out that the success of this system requires the thresholds for successive developmental stages to be widely separated, which precludes species with high thermal coefficients for growth. Therefore i t i s l i k e l y to be developed only i n summer species which have a slow rate of growth (i.e. low thermal coefficient for growth). Consequently, Corbet has divided summer species into two additional categories: (a) those which require two or more years to complete a generation, and (b) those which require only one year. Corbet, believes that these categories represent stages i n the colonisation of the higher latitudes from the tropics. He regards spring species as the most highly adapted to high latitudes. - 3 -Summer species of type (a) also show adaptation to temperate climates, while type (b) appears least modified from the presumed ancestral tropical type. Corbet's scheme contains several untested premises. F i r s t , he assumes that spring species have low thermal coefficients for growth compared with summer and tropical species. He makes this assumption because spring species generally inhabit permanent and r e l a t i v e l y cold water, i n which high thermal coefficients for growth have l i t t l e value, whereas tropical species often occupy temporary habitats subject to rapidly r i s i n g temperatures (Corbet, 1963). Summer species should have thermal coefficients for growth which f a l l between those of spring and tropical species. Second, Corbet (loc. c i t . ) believes that spring species should exhibit the greatest tolerance of low temperature, beginning with the f i n a l instar (the f i r s t stage to be overtaken by winter), and progressively extending to the earlier instars. I propose that superior tolerance of low temperature w i l l be reflected by higher growth rates at low temperatures, as well as rel a t i v e l y low temperature thresholds for metamorphosis, emergence, and the Induction of diapause or quiescence. Finally, I suggest that spring species can be expected to show the most general u t i l i s a t i o n of, and the most highly developed responses to changes i n photoperiod. I propose this hypothesis because the extensive and systematic variation i n photoperiod at high latitudes makes i t a suitable factor by which to regulate development. The purpose of this study was to test the following hypotheses: (1) spring species have lower thermal coefficients - 4 -for growth than do summer species; (2) spring species are more tolerant of low temperatures than are summer species, which i s shown by higher growth rates at low temperatures, and (3) spring species show greater u t i l i s a t i o n of photoperiod In regulating development than do summer species. Three species were chosen for the study: the Zygopteran Enalla^ma boreale (Selys) and the Anlsopterans Leucorrhinia  g l a c l a l l s (Hagen) and L i b e l l u l a quadramaculata (Linne). E. boreale i s a summer species i n terms of Corbet's c l a s s i f i c a t i o n , while the Anisopterans are both spring species. No tropical species were included i n the study. MATERIALS AND METHODS. Field studies were carried out at Marlon Lake, situated i n south-western B r i t i s h Columbia on the southern slopes of the Coast Mountains (49 19'N, 122 33'W-; a l t . 305 m). Marion Lake i s located i n the University of B r i t i s h Columbia Research Forest, about 50 km north-east of Vancouver. Marion Lake l i e s within the l i t t o r a l climatic region, but the lake's typi c a l l y mild climate i s modified by the Coast Mountains i n having a heavy winter, r a i n f a l l (Efford, 1967). The average annual r a i n f a l l i s 94 inches, 10% of which f a l l s between October and March (Efford, loc. c i t . ) . The average annual temperature i s 8.8 C, with a lowest mean monthly temperature of 1.7 C (January) and a highest of 16.2 C (July) (Efford, loc. c i t . ) . Marion Lake i s small and shallow, approximately 800 m long and 200 m across at the widest point, with an average depth of 2.4 m (Efford, 1967). The bottom i s comprised mostly of soft mud and over much of i t s area supports l i t t l e vegetation. Beds of Nufehar--. varlegatum, Potamogeton natans. Potamogeton eplhydrus and Equisltum sp. occur near the shore. Areas of Isoetes  ocoldentalis are found i n deeper water and patches of Chara  globularis occur i n the large springs i n the lake. Sedges and small shrubs are numerous along the shore. The odonate nymphs were generally found i n association with the vegetation, either on the stems of the plants, or in the decaying material around the roots. Few nymphs were obtained from the open sediment. The nymphs used i n the rearing experiments were obtained b'yosweeplng the vegetation and substrate with a hand-net. The samples were sorted i n the f i e l d . No attempt was made to obtain quantitative samples because only Enallagma boreale was suf f i c i e n t l y abundant to justify the effort required. The timing and patterns of emergence were determined by collecting exuvia at weekly intervals from the emergent vegetation. Six areas of approximately 4 m were located around the lake for this purpose (Figure 1). The laboratory studies were carried out at the University of B r i t i s h Columbia. The nymphs were reared i n photoperlods of 6, 9, 12, 15 and 18 hours at 10 C, 15 C, 20 C and 25 C ( a l l ± 1.5 C). Four 1.22 m X 1.22 m X 0.23 m boxes were b u i l t , each divided into 1.22 m long light-proof compartments which were inter-connected to permit the flow through of water. The water was circulated through the boxes from 800-litre reservoirs at approximately 2 l i t r e s per minute. Each compartment was f i t t e d with one 1.22 m standard "cool-white" fluorescent lamp with ballast and housing. Five "Intermatic" single-pole, single-throw - 6 -Figure 1. Marion Lake: location of the s i x sites (numbered J.-6) from which the exuvla were collected. Outlet - 7 -time-switches were used to regulate the photoperiods. Each time-switch controlled four lamps, one from each box. The nymphs were reared singly i n 9.5 cm diameter X 7.0 cm deep transparent plastic containers, which were floated i n the water circulating through the boxes. The water i n the containers was renewed at 3-4 week intervals. The nymphs were fed White worm (Enchaetraids) by hand once per day u n t i l satiated. A l l were i n i t i a l l y fed daily, but later the 15 C nymphs were reduced to three feedings per week, and the 10 G .nymphs to two feedings per week. Records were kept of food consumption by representative nymphs. The nymphs were checked for ecdysis, emergence and deaths at the time of feeding. The development of the eggs was also studied, using the apparatus i n which the nymphs were reared. The eggs of the two exophytic (oviposltlon) species, Leucorrhlnia g l a c i a l l s and L l b e l l u l a quadramaculata, were obtained by capturing laying females and inducing them to continue laying. This was done by dipping the t i p of the abdomen in water. Females of the endophytic Enallaftma boreale were provided with plant stems on which to lay. ORIGINS', DISTRIBUTIONS AND PATTERNS OF EMERGENCE. Seven species of Odonata are known from Marion Lake. These are the Anisopterans Plathemis lydla (Drury), Leucorrhlnia  g l a c i a l l s (Hagen), L l b e l l u l a quadramaculata (Linne), Aeshna  Interrupt a (Walker), and the Zygopterans Enallapjna boreale (Selys), Ischnura cervula (Selys) and Lestes dryas (KIrby). The adult of a f i f t h Anisopteran has been observed on the wing, but has not been identified. - 8 -The feature of the ©donate fauna of the southern coastal region of B r i t i s h Columbia, as exemplified by the species found at Marlon Lake, i s the overlapping of boreal and austral faunas, which i s much more marked than i n any other part of Canada. This appears due to the cool summers of this region, which permit southward extension of boreal species, and the long summers and mild winters, which permit the existence of certain austral species. L. g l a c i a l i s . L. quadramaculata, A. lnterrupta e L. dryas and E. boreale belong to the General Boreal fauna (Walker, 1927), and a l l are widely distributed i n the Hudsonian and Canadian Zones (see Klugh and McDougall, 1924). These species a l l have trans-continental distributions. In addition, L. dryas and L. quadramaculata are circumpolar i n distribution. P. lydla and I. cervula belong.to the Au3tral fauna (Walker, 1927). P. lyd l a inhabits the Transition and Upper Austral Zones and hence has an interrupted distribution i n Canada. I. cervula belongs to the Western Austral or Sonoran fauna and i s therefore restricted to southern B r i t i s h Columbia and Western Alberta i n Canada. From the standpoint of origin, the Marion Lake species, other than I. cervula. belong to holarctic genera, or to holarctic sections of genera (Walker, loc. c i t . ) . I. cervula represents a nofcthward extension of the Sonoran fauna. E. boreale. L. g l a c i a l i s and L. quadramaculata are therefore a l l boreal i n distribution and holarctic i n origin. E. boreale i s the most widely distributed species of Enaliagma i n Canada and i s the commonest Enallagma i n the far north (V/alker, 1953). In fact, E. boreale Is probably the most abundant Odonate In the northern part of the continent, and i t may also range farthest north. It has been recorded from Palmer, Alaska (61 30'N); Dawson, Yukon (64 N) and Fort Norman, North West Territories (61 N) (Walker, loc. c i t . ) . The records are not sufficient to compare L. g l a c i a l i s and L. quadramaaulata, but i t i s clear that both species are abundant at high latitudes. L. quadramaculata is generally distributed through Canada and Alaska, except i n the Arctic Zone (Walker, 1927). Walker records that i t was the commonest of the few species of Anlsoptera that occurred at Prince Rupert, where L. g l a c i a l i s was also common. E. boreale i s the earliest of the Marlon Lake species to ©merge (Table 1). In 1969 and 1970 emergence began about the middle of May and continued u n t i l mid-August. With L. p;lacialls It has the longest emergence period of the species at Marion Lake. The unsynchronised pattern of emergence, resulting from the lack of a f i n a l instar diapause, shows that E. boreale i s a summer species. L. quadramaculata typi c a l l y begins emerging two weeks after E. boreale, i n late May or early June. Emergence i s restricted to about 30 days (Table 1), and more than 75$ of the annual population emerges In the f i r s t week of the emergence period (1969 and 1970). The close synchronisation of emergence i s the result of the diapause in the f i n a l instar. L. quadramaculata therefore has a l i f e history of the spring type. L. PuLaclalis begins emerging at the same time as L. quadramaculata. Except for E. boreale. these two species are the earliest to emerge of the Marlon Lake species. The pattern of - 1 0 -1262. Day/Month. 12/5 18/5 14/5 3/6 11/6 21/6 1/7 8/7 18/7 27/7 6/8 22/8 29/8 6/9 L. quad. 114 13 4 1 - .3 - - - - - -L. glac. - - 50 14 43 5 1 4 2 - . 4 23 32 19 E, boreale. to 110 18 123 46 3 4 50 16 4 1 2 — [970. Day/Month. 1 13/5 23/5 30/5 6/6 13/6 20/6 4/7 11/7 19/7 1/8 9/8 15/8 22/8 12/S L. quad. 2 24 3 2 1 CO L. glac. » .- mm mm 5 2 1 1 1 4 1 28 23 27 15 E. boreale. 132 80 77, 44 45 59 67 47 2 5 - - C O 1 971. Day/Month. 23/5 30/5 5/6 12/6 20/6 28/6 5/7 12/7 19/7 7/8 16/8 23/8 L, quad. - 2 1 - 1 - _ 2 Ii. Rlac. - 2 2 1 mm 3 5 11 15 8 E. boreale. - 85 133 29 46 17 49 41 175 1 76 7 1 Table 1. The patterns of emergence of S. boreale. L. gl a c i a l l s and L. quadramaculata. Each figure is the number of exuvla collected from an area of 24 in" in the interval prior to the date given. - 11 -emergence of L. g l a c i a l l s i s quite different from those of either of the other two species. It i s characterised by two peaks, one at the beginning of emergence, and the second about two months later. Because a (facultative) diapause occurs i n the f i n a l Instar, L. g l a c l a l i s i s presumed to be a spring species. THE REARING EXPERIMENTS Table 2 presents the growth rate data obtained for the three species under the various combinations of temperature and photoperiod. The data were obtained from fourth to penultimate (12th or 13th) instar nymphs. Individual nymphs were reared for periods ranging from a minimum of a month at 23 C and 20 C, to 14 months at 15 G and 10 C. Each figure, excluding the means of a l l observations, i s the mean time i n days taken to complete two ecdyses at the specified temperature and photoperiod. The unbracketed values were calculated from the observed number of days taken to complete two ecdyses. The bracketed values for L. g l a c l a l l s and L. quadramaculata were calculated from the t o t a l number of ecdyses that occurred during the pooled time the nymphs were reared. This procedure was necessary because at 10 C and 15 0 many of the nymphs of these two species f a i l e d to complete two ecdyses during the study. Where both bracketed and unbracketed values are presented, the former are more representative because they account for every nymph, including those which f a i l e d to complete one ecdysls during the experimental period. The effect of temperature on the growth rate. The mean of the t o t a l number of observations at each temperature was used as the measure of the growth rate response - 12 -E. boreale L. gl a c i a l i s L. quadramaculata 25 C 18 hr. 11.5 15 14.1 12 12.4 9 12.7 6 14.4 Mean of a l l observ. 13.0 23.3 18.9 20.8 21.1 26.9 20.1 20.1 16.3 2774 20 G Mean 18 hr. 15 12 9 6 of a l l observ. 25.4 20.3 17.6 17.1 26.5 33.3 24.2 26*79 31.1 23.9 27.5 20.3 2o73 15 C 18 hr. 34.3 15 24.1 12 29.1 9 31.7 6 30.5 Mean of a l l observ. 29.9 83.O (81.2) 104.7(100.5) 122^6(148.4) 10375(105.9) 104.8(111.1) (513.0) (195.7) 10 C 18 hr. 85.4 15 52.8 12 66.9 9 60.3 6 64.8 Mean of a l l observ. 63.8 147.4(139.3) 169.2(252.7) 15.27.0? [229.2) 162.3(243.5) (513.0) (333.57 Table 2. The growth rate data obtained for E. boreale, L. g l a c i a l i s and L. quadramaculata. Each figure, excluding the means of a l l observations, is the mean time in days taken to complete two ecdyses at the specified temperature and photoperiod. Where both bracketed and unbracketed values are presented, the former constitute the basis of the analyses (see page 11). to temperature (Table 2). Equal numbers of equivalent Instar nymphs were reared at each photoperiod In order to reduce biases caused by response to photoperiod. Table 3 gives for each temperature the ratios of the time taken by each species to complete an instar relative to the time taken by E. boreale. It i s clear that E. boreale develops much more rapidly than either L. g l a c l a l i s or L. quadramaculata at each of the temperatures tested. The difference is most apparent at 15 C and 10 C, where E. boreale developed between 3.5 and 6.5 times more rapidly than the other two species. There were no detectable differences i n growth rate between L, sflaclalis and L. quadramaculata at 25 C and 20 G, but at 15 G and 10 G L. relaclalis required l i t t l e more than half the time needed by L. quadramaculata to complete one instar. Discussion. The data presented i n Tables 2 and 3 support the hypothesis that summer species have higher temperature coefficients for growth than spring species. E. boreale, the summer species, developed more rapidly than either L. g l a c i a l l s or L. quadramaculata at a l l of the experimental temperatures. On the other hand, the data do not support the proposal that spring species are more tolerant of low temperatures than summer species. I f superior growth rate i s taken to indicate superior tolerance of low temperatures, E. boreale i s clearly the most tolerant of the three species. Corbet (1956, 1963) believes that spring species represent the f i n a l stage i n adaptation to cold climates. However, the - 14 -E. boreale L. g l a c i a l l s L. quadramaculata 25 G 1 : 1.6 : 1.6 20 G 1 : 1.4 : 1.4 15 G 1 : 3.5 : 6.5 10 G 1 : 3.6 : 5.2 Table 3. The growth rate responses of L. pJLacialis and L. quadramaculata to temperature expressed as ratios relative to the growth rate of E. boreale. Temperature. 15 C 10 G Photoperiod - hr. 18 15 12 9 6 18 1 5 1 2 9 6 L. g l a c l a l l s 1 : 1.2 : 1.8 1 : 1.8 : 3.8 L. quadramaculata 1 : 4.5 1 : 2.1 Table 4. The growth rate responses of L. Rl a c i a l i s and L. quadramaculata to photoperiod expressed as ratios relative to the fastest growth rate shown by each species. - 15 -laboratory results leave l i t t l e doubt that E. boreale i s better able to develop at low temperatures than are L. g l a c l a l l s and L. quadramaculata. Field observations agree with .the laboratory results. E. boreale i s the f i r s t species to emerge at Marion Lake and i t i s probably the most abundant of the three at high latitudes. The tolerance to low temperature demonstrated i n E. boreale i s not necessarily a refutation of Corbet's scheme, even though other unusually tolerant summer species probably exist (e.g. Enallagma cyathlgerum Charp). The results of this study are interpreted to mean that there may have been several pathways of adaptation to high latitudes. The more common pathway, proposed by Corbet, was the development of the diapause stage, possibly by the physiologically less tolerant and adaptable species. The other pathway could have involved the development of greater tolerance, presumably by the physiologically most adaptable species. The effect of photoperiod on the growth rate. Either an analysis of variance ( L i , 1964), or the Mann-Whitney U test (Siegal, 1956) was carried out on the photoperiod data i n Table 2. The analysis of variance was applied to the data at each of the four temperatures for E. boreale, and to the 25 C and 20 C data for L. Fqacialis and L. quadramaculata. The Mann-Whitney U test was performed on the 15 C and 10 C data obtained for L. g l a c l a l l s and L. quadramaculata. There was no significant difference (99$ confidence limita) between photoperlods for any of the species at 25 C and 20 C. This was also true for E. boreale at 15 C and 10 C. On the other hand, - 16 -both L. g l a c l a l i s and L. quadramaculata showed significant responses {95% confidence limits) to photoperiod length at 15 G and 10 C. Discussion, I predicted that summer species would show relatively-l i t t l e adaptation or response to photoperiod compared with spring species. I proposed this hypothesis because summer species, having higher thermal coefficients for growth, and l i v i n g i n re l a t i v e l y warm habitats, have l i t t l e need of response to photoperiod i n regulating growth. The data presented i n Tables 2 and 4 support this contention. E, • boreale showed no detectable growth rate response to photoperiod at any of the temperatures tested, while the spring species showed well-developed responses. The.fact that L. glacialIs and L. quadramaculata responded to photoperiod only at the lower two temperatures i s interpreted as additional support for the hypothesis that low temperatures favour the development of photoperiod response. Both spring species showed a greater response to photoperiod at 10 G than at 15 C, which i s also Interpreted as support for the above hypothesis. The difference in response i s most readily seen i n L. g l a c l a l i s (Table 4J , for which the rati o between the shortest and the longest photoperiods was more than twice as long at 10 C (1 : 3.8) as i t was at 15 0 (1 : 1.8). L. quadramaculata i s evidently more responsive to photoperiod than L. p;lacialis at equivalent temperatures. This can be seen best at 15 C, where L. quadramaculata produced a growth rate r a t i o of 1 :.4.5 for a 6 hr Increment i n photoperiod, whereas L. g l a c i a l l s produced ratios of 1 : 1.2 and 1 : 1.5 for increments of the same length (Table 4). 4 rr - U -The trends discussed i n the previous two paragraphs are not immediately apparent at 10 C, bufc can.be demonstrated by the following analysis. L. g l a c i a l i s developed as rapidly at 10 G 18 hr as at 15 G 6 hr (Table 2). However, L. quadramaculata required less than half the time for development at 10 C 15 hr (243.5 days) as at 15 G 9 hr (513.0 days).. The photoperiod Increment was 12 hr for L. g l a c l a l i s , but only 6 hr for L. quadramaculata. There can be l i t t l e doubt that L. quadramaculata has, on the one hand,, a lower thermal coefficient for growth than L. g l a c l a l i s , but i s more responsive to photoperiod. While there i s the danger of over-interpreting the data, this conclusion agrees with the hypothesis that the higher the thermal coefficient for growth, the less developed the response to photoperiod. To conclude, comment should be made of the relative influence of photoperiod and temperature on the growth rate. It has been shown for L. quadramaculata that a 6-hr increase i n photoperiod has, at low temperatures, a substantially greater effect on the growth rate than a 5 C rise i n temperature. Even i n the less responsive L. p l a c l a l l s , a 12-hr increase in photoperiod i s as effective as a 5 C rise in temperature. These results suggest that in habitats which experience long periods of low temperatures, photoperiod may be a dominant factor i n determining growth rate. The response of the f i n a l instar to the experimental conditions. The f i n a l instar could not be studied i n a systematic manner with the apparatus used for this study, but several observations are worth comment. The f i n a l instar has to be - 18 -studied separately because the development and duration of this instar i n some species i s evidently affected by complex Interactions of temperature and photoperiod which have l i t t l e bearing on earlier development. E. boreale emerged at a l l four temperatures, with no detectable differences i n success. Temperature appeared to affect the duration of the f i n a l instar In the same manner i n which i t affected the earlier instars. Similarly, photoperiod did not appear to affect the time spent i n the f i n a l instar. Most of the L. g l a c i a l i s nymphs reared at 25 C and 20 C reached the f i n a l Instar but subsequently f a i l e d to emerge. This suggests that diapause i s obligatory. On the other hand, f i e l d evidence suggests that the diapause i s facultative. For example, a large number of the f i n a l Instar were present i n Marion Lake prior to the second peak of emergence (Table 1), but many did not take part i n this emergence. Corbet (1955, 1956, 1957) showed that Anax imperator. which has a pattern of emergence l i k e that of L. g l a c i a l l s . has a facultative diapause i n which the instar forgoes diapause i f the day length Is increasing by more than two minutes per day. It seems l i k e l y that L. g l a c l a l i s responds i n the same way to changing day length. In other words, the constant conditions i n the laboratory probably predisposed the nymphs to diapause, despite apparently being otherwise favourable (e.g. high temperatures and long photoperiods). Two emergences were recorded at 15 C, and none at 10 C. Many of the nymphs which reached the f i n a l instar later died, apparently after having completed metamorphosis. Representative nymphs were removed from both temperatures and reared at higher - 19 -temperatures under natural photoperiods. Most of these quickly-emerged. It seems probable that temperature was the factor l i m i t i n g emergence at 15 C and 10 G i n the laboratory. In order to reconcile these results, we must assume that photoperiod i s c r i t i c a l i n i n i t i a t i n g diapause, but has l i t t l e bearing on emergence once diapause development', has been completed. One emergence was recorded for L. quadramaculata at 15 G. It i s l i k e l y that this species i s also unable to emerge at 15 G and 10 C. However, very few nymphs reached the f i n a l instar before the study was terminated. It i s also probable that diapause i s obligatory i n this species. This i s suggested by the presence of the f i n a l instar i n Marlon Lake throughout the summer following emergence. The response of the eggs to the experimental conditions. The eggs were studied with the expectation that they (1) would show thermal coefficients for development paralleling those of the nymphs and (2) would respond to photoperiod length. The eggs of L. g l a c i a l i s and L. quadramaculata obtained before the summer solstice hatched rapidly at high temperatures, with the l a t t e r species showing the s l i g h t l y faster rate of development. L. quadramaculata hatched i n approximately 8, 16 and 41 days at 25 C, 20 C and 15 G, and L. g l a c l a l l s required 10, 20 and 52 days. Both species f a i l e d to hatch at 10 C. Photoperiod did not affect development in either species. The eggs of L. g l a c l a l l s collected after the summer solstice took 4 - 7 months to hatch, i n contrast to the rapid development of the eggs collected earlier. In addition, the eggs at 10 G hatched. It appears that L. g l a c l a l l s has a facultative diapause - 20 -i n the egg as well as i n the f i n a l instar. The experiments with the eggs of E. boreale were unsatisfactory. Most f a i l e d to hatch, probably because of the premature decay of the plant material in'which they were l a i d . The hatch obtained suggests that developme&t i s slower i n this species (i.e. *v 21 days at 25 C). The effect of photoperiod could not be assessed. The results do not support the hypothesis that the eggs respond to temperature and photoperiod i n the same manner as do the nymphs. .For example, the thermal coefficients for development are reversed for L. g l a c i a l i s and L. quadramaculata i n the egg stage. Furthermore, both species probably develop more rapidly than E. boreale. Finally, the eggs of the spring species showed no response to photoperiod, i n contrast with their nymphs. There are several explanations for these results. At the species l e v e l , the differences can be attributed to the habitats i n which the eggs develop. The two spring species lay i n shallow water, which i s subject to drying up and, consequently, to very high temperatures. In this situation, high thermal coefficients for development are an important adaptation for survival. On the other hand, E. boreale lays i n plant stems i n deeper water, where there i s l i t t l e danger of drying out, and where temperatures are lower. In general, the eggs experience a narrow range of conditions (usually Involving high temperatures and long photoperiods) compared with the nymphs. Because the habitat i s different, the eggs require a different set of responses from those developed by the nymphs. CONCLUSIONS Several criticisms of the execution of the study should be noted. The object of the study was to compare summer and spring species. However, while one summer species and two spring species were studied, a l l three species were boreal i n distribution and holarctic i n origin. In other words, E.. boreale i s not a typical summer species, at least i n distribution, because i t occurs at higher latitudes than would be expected of a summer species. E. boreale was included i n the study with the expectation that i t would be d i f f i c u l t to categorise. However, the study consequently lacked a "typical" summer species. Plathemls lydla and Ischnura cervula were obvious choices, but neither was suf f i c i e n t l y abundant to work with. In the event, E. boreale proved informative, but several more typical summer species would complete the comparison. The experimental environment i n which the nymphs were reared was not entirely satisfactory. The static system used was subject to the accumulation of waste products, and the growth of bacterial and algal populations. The dissolved oxygen supply undoubtedly declined at the same time. Periodic renewal of the water reduced the problem to some extent, but the process was laborious, and of course did not maintain constant conditions. It was not possible to determine whether the growth rate of the nymphs was affected by the experimental environment, but i t i s assumed that the more constant environment produced by a continuous flow system would at least reduce experimental variance. When assessing the effects of the experimental conditions on the development of the nymphs, we should consider the conditions - 22 -previously experienced i n the natural habitat. This i s particularly important In the case of the f i n a l instar, which may have been conditioned to diapause prior to removal from the f i e l d . This study did not compare growth rates of earlier instars obtained before and after the summer solstice, nor on either side of the autumnal equinox, both apparently important times i n f i n a l Instar development (Corbet, 1963; Lutz. and Jenner, 1964). It Is assumed that the development of a l l but the f i n a l Instar, depending upon the species, Is facultative, but this has to be proved. The photoperiodic reactions of insects and mites characteristically control alternative pathways of development, such as diapause or form determination (Lees, i960). The response i s therefore of an all-or-nothing nature, involving a c r i t i c a l photoperiod. For example, i n Megoura vicae the transition from a short day to a long day effect takes place over less than 30 minutes, as the photoperiod i s extended from 14§ to 15 hours (Lees, loc. c i t . ) . A very few insects have been shown to respond to the progression of photoperiod. The response of the f i n a l instar of Anax imperator to changing photoperiod has already been mentioned. Response of this kind to changing day length has been demonstrated only i n two other arthropods, a locust, Nomadacrls septemfasciata, and a beetle .Anthremus verbascl (in Corbet, 1963). The present study suggests that both the adult and the f i n a l instar of L. g l a c l a l i s are also capable of responding to changing photoperiod. The results reported i n this study are of interest because they suggest that at least one Invertebrate (i.e. L. g l a c l a l i s ) - 23 -i s capable of continuously variable responses to changing photoperiod. Unlike normal invertebrate reactions, this response seems similar to the photoperiodic control of testicular growth i n birds, where l i t t l e growth occurs at photoperiods of . less than eight hours, but which increases to a maximum in continuous l i g h t (Lees, 1960). Several other species of Odonata appear l i k e l y to respond to photoperiod i n the manner demonstrated for L. g l a c i a l i s . L. quadramaculata i s l i k e l y to do so. Lutz (1968) has shown that the f i n a l four lnstars of Lestes eurinus grow more rapidly at a 14-hr photoperiod than at 11 hr. Preliminary work by Jenner (1969) showed that some of the later instars of several species of Odonata also developed more rapidly at 14 hr than at 11 hr. However, the presence of this response must be confirmed by testing the effects of at least three photoperiods on the rate of development, otherwise the responses may be attributed to the presence or absence of diapause. It i s clear that a number of Odonata are highly responsive to changes i n photoperiod. This study shows that the influence of photoperiod i s not necessarily restricted to the f i n a l instar, or to the egg (through the adult) but, In some species, may affect the rate of development throughout the l a r v a l stage. Furthermore, evidence presented i n this study shows that photoperiod can have an extraordinary influence on the rate of development at low temperatures, to the extent that photoperiod may be the dominant factor i n regulating development at low temperatures In some species. In view of these considerations, i t i s evident that the photoperiod responses of Odonata of high latitudes warrant further study. - 24 ACKNOWLEDGEMENTS This study was supported with grants from the Canadian International Biological Program. I wish to express my appreciation of the help, and patience, shown me by my thesis supervisor, Dr. I. E. Efford. I also wish to thank Dr. D. Chltty, who provided helpful critism of the manuscript, and Mrs Anthea Bryan and Mr. Michael Hoebel, who took care of experiments i n my absence. Mrs. Kathryn Calef, Mr. Kanji Tsumura and Mr. Itsuo Yasaki provided technical assistance. Finally, I wish to thank my wife, Mrs. Anita Procter, who helped without complaint throughout the study„ - 25 -REFERENCES Calvert, P. P. (1934). The rates of growth, l a r v a l development and seasonal distribution of dragonflies of the genus Anax (Odonata: Aeshnidae). Proc. Amer. P h i l . Soc. 73 : 1-70. Cloudsley-Thompson, J. L. (1961). Rhythmic ac t i v i t y i n animal  physiology and behaviour. Academic Press. Corbet, P. S. (1955). A c r i t i c a l response to changing length of day in an insect. Nature, London 175* 338-^39. _ . . (1956). Environmental factors influencing the induction and termination of diapause i n the Emperor Dragonfly, Anax inroerator Leach (Odonata: Aeshnidae). J. exp. B i o l . 33: 1-14. . (1957). The l i f e - h i s t o r y of the Emperor Dragonfly leach (Odonata: Aeshnidae). J. Anim. Ecol. 26: 1-69. • (1958). Temperature i n relation to seasonal development - -of B r i t i s h dragonflies (Odonata: Aeshnidae). Proc. X Int. Congr. Ent., Montreal 2: 755-757. Corbet, P. S., C. Longfleld and N. W. Moore. (i960). Dragonflies. Collins. . Corbet, P. S. (1963). A biology of dragonflies. Quadrangle Books. Efford, I. E. (1967). Temporal and spatial differences In phytoplankton productivity i n Marion Lake, B r i t i s h Columbia. J. Fish. Res. Bd. Canada 24: 2283-2307. Hodgkin, E. P., and J. A. L. Watson. (1958). Breeding of dragonflies i n temporary waters. Nature, London 181: 1015-1016. Jenner, C. E. (1958). The effect of photoperiod on the duration of nymphal development In several species of Odonata. Quart. Pubn. Ass. S. B i o l . , Philidelphia 6: 16. Klugh, A. B. and E. G-. McDougall. (1924). The Faunal Areas of Canada. Handbook of Canada, B r i t i s h Association Meeting, Toronto. Lees, A. D. (1955). The physiology of diapause i n arthropods. Cambridge University Press. . (1960). Some aspects of animal photoperiodism. Cold. Spr. Harb. Symp. Quant. B i o l . 25: 261-168. - 26 -Lutz, P. E. and C. E. Jenner. (1964). L i f e - h i s t o r y and photoperiodic responses of nymphs of Tetragoneura  cynosura Say. B i o l . B u l l . 127~« 304-3To^  Lutz, P. E. (1968). Effects of temperature and photoperiod on l a r v a l development i n Lestes eurinus (Odonata: Lestidae). Ecology 49: 637-644*^  ' . Walker, E. M. (1927). The Odonata of the Canadian C o r d i l l e r a . The P r o v i n c i a l Museum of Natural History, V i e t o r i a , B r i t i s h Columbia. • • (1953). The Odonata of Canada and Alaska. Vol. 1. - . - .University of Toronto Press. . (1958). The Odonata of Canada and Alaska. Vol. 2. U n i v e r s i t y of Toronto Press. Whitehouse, F. C. (1941). B r i t i s h Columbia dragonflies (Odonata) with notes on d i s t r i b u t i o n and habits. Amer. Mid. Nat. 26: 488-557. 

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