lUti 



KNOWLEDGE, 



[SXPTEMBER, 19U2. 



clKiiifffil throiifj^h N.W., N., !ind N.E. witliin the limits of 

 the iiietrojiolis. I would submit that these viining currents 

 are the rule over London. 



THE NUMBER OF MOULTS UNDERGONE 

 BY DRAGON-FLY NYMPHS. 



Uy the Kev. jVbtiiuk East. 



Theke a])iJear to be so few observations of the number of 

 times that the nymphs of dragon-flies change their skins 

 — indeed, so far as the writer is aware, the exact number is 

 unknown — that possibly a few notes on the ecdysis of 

 JEsvhna cyaiiea may be acceptable to that small, but 

 increasing number of students who devote some attention 

 to that interesting group of insects known as the Odonata. 

 The moults in the later stages of the nymph's growth 

 are very easily observed and counted, but it is the earlier 

 ones, and especially the earliest of all, where the difficulty 

 comes in. This, of course, is owing mainly to the insects 

 being aquatic in the nymphal stage, and also to the 

 minuteness of the eggs of even the largest dragon-flies ; 

 added to this there is the difficulty of keeping the 

 imagines in captivity so as to make the females deposit 

 their eggs where tliey can be kept under observation, and 

 the additional fact that when the minute eggs are laid, 

 they are either secreted beneath the cuticle of aquatic 

 plants or else dropped at random into the water. Accident, 

 however, comes sometimes to the rescue. 



There was found in an aquarium in July an ex- 

 cessively minute and solitary nymjih of ^schiia cyanea, 

 measuring only 65 mm. in length, and in addition there 

 were three still more minute cast nymph skins, measuring 

 resjjectively 3i mm., 5 mm. , and 6^ mm. , evidently belonging 

 to this same nymph, as the water had been unchanged 

 siuce January. The smallest skin, measuring 3^ mm., 

 was possibly the first or second skin cast. A writer in the 

 December number of the Entomologist, referring to this 

 particular instance, says " A very few minutes after 

 leaving the egg, some young nymphs moult. This I have 

 observed in Lihellula qnadrimacutata (at Jena, Germany, 

 in June, 1896) and in our American L. pulchella and 

 Sympetrum vicinum. It is also mentioned for Epitheca 

 bimaculata by Heymons (1896). It is likely that such a 

 moult will be found to occur in yEschna. Previous to 

 this first moult, the legs of the young nymph are adherent 

 to each other and are not movable, so that Heymons 

 says that we -cannot yet speak of a larva, but of a 

 hatched embryo. The first moult frees the legs, which are 

 at once put into use by their possessor. 



The day following that on which the nymph was first 

 discovered it changed again, and became 10 mm. in length; 

 on Augsut 3rd, it became 13 mm. ; on August 15th, it 

 became 18 mm. ; and at this, its sixth 

 moult, the four rudimentary wing-cases 

 first appeared. 



The seventh moult was on August 25, 

 when the nymjih measured 20 mm , the 

 rudimentary wiugs were further deve- 

 li>jifd, and, strangely, there could be seen 

 thriiugh the transparent skin the wing- 

 cases of the next moult, now for the first 

 time advanced from the sides and lying 

 touching one another on the median line 

 of the back. 



Then followed a change on Se])tein- 

 ber 22, when the nymph measured 25^ 

 mm., and after this a long rest of 

 four months to January 24, when the measure became 



Nymph of ^schna 

 cyanea after the 

 lith moult, sliow- 

 ing first appear- 

 ance of wing- 

 cases. 



29 mm. Unfortunately during the winter this nymph 

 died, and there had to be substituted ^. cyanea II., 

 measuring, however, exactly 29 mm., three others of the 

 same measure being kept in reserve in case of disaster. 

 No disaster occurred, happily, and JE. cyanea II. com- 

 pleted her course (for the nymph was a femcale) without 

 accident. The subse^iuent changes and measurements 

 were as follow: — 10th moult. May Sth, 32 mm:; 11th 

 moult, June 6th, 38 mm. ; 12th, final emergence of the 

 imago, July 14th, the nymph having reached 43 mm. 



The number of moults undergone will therefore lie twelve, 

 sujiposing that the smallest skin found was that of the 

 first moult, if of the second moult then the total number 

 must be advanced one, making thirteen as the number. 



For convenience of reference the whole observation may 

 be given in tabular form — 



? 1st moult, tUt« iinknonn, measure .3i mm. 



2nd moult, date unknown, measure 5 mm. 



3rd moult, dale imknown, measure 6i mm. 



4tli moult, July Slst, measure 10 mm. 



5th moult, August 3rd, measure 13 mm. 



6th moult, August 15th measiu-e 18 mm. 



7th moult, August 25th, measure 20 mm. 



8th moult, September 22nd, measure 25^ mm. 



9tli moult, January 24th, measure 29 mui. 



On April 30th, substituted another nymph in place of 



original one, which died. 



10th moidt, May 5th, measure 32 mm. 



11th moult, Jmie 6th, measure 38 mm. 



12th and final, July 14th, nymph measured 43 mm. 



From this it will be seen that there is a fairly even 

 growth, on an average, of 4 mm. for each ecdysis, but that 

 for nearly eight months, viz., from September 22nd to 

 May 5th, there was only one moult. This was not owing 

 to lack of food, as worms were frequently offered, nor was 

 the nymph at all torpid ; one can only suppose that it is 

 a necessary arrangement, as food must ordinarily be 

 extremely difficult to obtain during the winter months. 



That the nymph should die during the observation was 

 unfortunate for both observed and observer, but the cause 

 showed very clearly the extreme value of that wonderful 

 endowment possessed by these creatures of being able to 

 breathe at will either the air dissolved in the water, or 

 common air at the surface, as referred to in a previous 

 article on this subject.* The small aquarium in which the 

 nymph was kept had been cleaned, and the sides were no 

 longer rough with green alga; as before. As the water 

 got stale the nymph would ordinarily have climbed the 

 side, and, protruding the rectum, have breathed the outer 

 air. The slippeiy sides, however, forbade this, and the 

 nymph died of asphyxia — breathing, 1 fear, maledictions on 

 the stupidity of its captor. 



The further point remains — which some reader can, 

 perhapis, clear up — as to when the eggs of ^. cyanea are 

 hatched. The imagines ajipear late in June and during 

 July, but ovipositing does not seem to begin until much 

 later in the year, the end of August or beginning of 

 September ; yet, even so, as late as July 30, a nymph only 

 measures 6i mm., leading one to suppose at first sight 

 that the eggs lie dormant during the winter and are 

 hatched when the warm weather begins ; but looking to 

 the fact mentioned above that during the second winter 

 the nymph scarcely grows at all, as evidenced by there 

 being one moult only between the end of September and 

 the beginning of May, we may perhaps suppose that the 

 young nymphs are hatched in autuur.i, but that they 

 grow but very little during the first winter. 



But whether the young nymphs are hatched in the 

 autumn or spring, it is clear that nearly two years are 

 required to perfect, the imago, viz., from the autumn of one 



* Kkowlkdob, October, 1900. 



