Larval life in Sarasaechna pryeri
thigmotactic response
Figure 1. All larvae reared in the container cling tightly to the leaf.
Preface

Sarasaeschna pryeri emerges in early spring (late April - early May), and oviposition usually takes place during from spring to early summer. In central Japan, its flight period is from late April to late July. However, we have very little knowledge on its larval stage as well as in other species among genus Sarasaeschna. Many investigators have tried to capture its living larva but very few of them have succeeded. Although its adults and exuviae can be found easily, it is very difficult to find the living larvae. They are hardly found in any pools around the spot where their exuviae are found. Where they hide, - this was one of puzzling problems among odonatologists.


Short History of Capturing Larvae of S. pryeri in Japanese Field

emergence
Figure 2. Left: Emergence of Sarasaechna pryeri, Right: The final stadium larva and an exuviae.
It is rather easy to find exuviae, but very difficult to encounter larvae although you search them in the small pools around the place where exuviae is found.

Taketo, in 1957, first discovered many exuviae of S. pryeri in Japan and reported on the larval habitat based on his finding (Taketo, 1958) although he could not capture any living larvae at that time. He mentioned that many exuviae were found clinging on the vegetations growing in a rather open swampy field covered with grasses and small bushes, and that it was quite reasonable for the larvae to live in the wet soil because the eggs were laid into the wet soil or mosses under a dense bush where water usually disappeared (Taketo, 1958).


In 1958, he finally succeded to find two living larvae (one in final, another in early instar) at the same spot where he observed the oviposition (Taketo, 1959). He mentioned that the larvae were captured from the surface-mud mixed with decayed vegetation which were accumulated in a tiny, 5-20 cm deep, stagnant water under a small bush.


Habitat
Figure 3. Left: A male perching in its territory. Right: A typical habitat of Sarasaechna pryeri. The arrow shows a territorial male (the left one) parching. Toyooka City, Hyogo Prefecture.

After that, in 1965, discovery of the final instar larvae (3 males and 1 female) in Kyoto was repoted (Obana, Inoue & Azuma, 1965). They mentioned on the habitat that all the larvae hid under the fallen leaves covering mud of the swampy but could not be found in the mud. In 1992, Arai et al. reported on their finding of some larvae in the field (Arai, Hirose & Suda, 1992). In November 1993, Fukui & Kasuya discovered 8 larvae in Shizuoka Pref., and, in December 1993, Fukui, Kasuya, Yoshida and I sought there for the larvae again and captured 56 individuals ! (Fukui & Kasuya, 1994). In January 1994, I caught a final instar larva from a small pool in the swampy at Nakamura City, Kochi.


Egg Stage and Hatching

oviposition
Figure 4. Oviposition of Sarasaechna pryeri into mud (left) and fallen leaves (right).

Many odonatologists have observed in the field that eggs are usually laid on wet soil without water, but no one knows the fate of the eggs. As far as eggs collected from female were reared under water, duration of egg stage extended 29-97 days (Arai, 1992) and 40 days (K.R.G.O., 1984). Embryo development of the latest ones seemed to stop in the stage just before hatching although the hatching seemed to be accelerated when such eggs were left for ca. 3 hours under dried condition (Arai, 1992).


Prolarva (1st instar larva) emerges out in a vermicular mortion through a slit of the chorion which is made by egg tooth (Kawashima, 1994). After emerging out, leaving the eggshell, it rises to the surface of the water and floats for 15- 30 minutes. As soon as the next moult occurs (15 - 30 minutes after hatching), the second instar larva moves to the bottom of the water (Kawashima, 1994).


Larval Stage

Some odonatologists had thought for the larva of S. pryeri to be terrestrial as it had been hadrly found in the pools. However, most larvae captured were in, or by, the water and, moreover, all larvae reared spent their stage under the water (Arai, 1992; Fukui et al., 1994; Kawashima, 1994). So, the larva is not terrestrial. Figure 3 (right) shows a typical habitat of the larvae and Figure 6 its section.


thigmotactic response and a figure of habitat
Left: Figure 5. If there is no substrate to cling, the larvae cling each other to be in a mass.
Right: Figure 6. Larvae inhabited small cavities of dried swampy. Water level was under ground. Larvae captured there usually clung under fallen leaves which were at the water level (green square). 30 December, 1993, at Kitazawa, Shizuoka Prefecture. (original).

The larva exhibits strong thigmotactic response. It also clings tightly to the leaf under rearing (Fig.7). When the leaf is removed, the larvae cling to each other to be in a mass (Fig.8).


The 56 larvae captured at Kitazawa included 7 instars (Fig.3). Their head width frequency histogram is shown in Figure 7.


the distribution of larval head width
Figure 7. Larval head width frequency histogram in S. pryeri at Kitazawa, Shizuoka Prefecture on 30 Decomber, 1993. Vertical dotted lines indicate means of each instar.

Duration of larval stage appear to vary. Under rearing condition, the shortest one was only one year from egg stage to emergence although it failed in emergence, but duration of the majority was 2 years (Fukui, 1996). Overwintering F-0 instar larvae probably emerge in the following spring. However, overwintering F-1 instar larvae do not always do so. Fukui (1996) mentioned that one (moulted on 5 April 1993) of three F-0 instar larvae which moulted from overwintering F-1 in spring emerged in the same spring, but two (moulted on 29 April and 28 May) of them did not emerge in the spring but in the following spring, that is, these two larvae spent the final instar stage for about one year. Photoperiod, temperature and/or food availability might be the factors which decide whether the final instar larvae could emerge in the same spring or not.


copulation and hovering
Left: Figure 8. Copulation of Sarasaechna pryeri.
Right: Figure 9. An adult of Sarasaechna pryeri hovering above the swampy.

Larval habitat of S. pryeri may be often subject to draught. Their wide variations of the duration of egg stage and of larval stage may be their adaptation to the habitat.


References

Arai, Y., Y. Hirose & S. Suda, 1992. Gekkan-Mushi 257:35. [-Jap]
Arai Y., 1992. Notes on the egg-period and immature larval charactors of Oligoaeschna pryeri Martin. Tombo 35(1/4):34-36.[-Jap. with Engl. summary]
Asahina, S., 1958. On the discovery and a description of the larval exuvia of Oligoaeschna pryeri Martin (Aeschnidae [sic]). Tombo 1(2/3):10-12.
Fukui, M., & M. Kasuya, 1994. Notes on the larval habitat of Oligoaeschna pryeri Martin. Aeschna 28:21-23. [-Jap]
Fukui, M., 1996. Notes on the larval stages of Oligoaeschna pryeri Martin. Aeschna 32:9-13.[-Jap]
Kawashima, I., 1994. Morphology and habits of prolarval and larval stage of Oligoaeschna pryeri (Martin) (Aeshnidae: Odonata). Nature and Insects 29(7):44-46.
Kansai Research Group of Odonatology, 1984. Dragonflies of Kinki District, Central Japan. [-Jap]
Obana S., K. Inoue & T. Azuma, 1965. Larvae of Oligoaeschna pryeri and their emergence. Gracile 2:1-4. [-Jap.]
Taketo, A., 1958. Some ecological observations on Oligoaeschna pryeri Martin (Aeschnidae [sic]). Tombo 1:(2/3)12-17. [-Jap. with Engl. summary]
Taketo, A., 1959. Discovery of the living larva of Oligoaeschna pryeri Martin (Aeschnidae [sic]). Tombo 2(1/2):2.