IV] THE LARVA OR NYMPH 69 



distinct, the legs very long. The hind end of the abdomen, 

 together with the cerci and the metatarsi, is bent round ventrally 

 so as to point forwards. The dorsal tracheal trunks, and strong 

 branches to the eyes, can be clearly seen, but are filled with liquid 

 instead of air. A large cylindrical plug of dark yellowish yolk 

 marks the limits of the mid-gut. 



Two or three days before hatching, the heart begins to beat, 

 with an irregular pulse of about 30 to the minute. It gradually 

 becomes more regular, and increases to 80 or more per minute. 

 A large vesicle (A, B, hv) forms between the head and the pedicel, 

 and appears to be filled with blood. Just before hatching, a very 

 distinct and peculiar pulsating organ, the cephalic heart (B-D, ch), 

 appears in the head, between the mouth and the aorta. This 

 organ is two-chambered, and pulsates like a simple two-chambered 

 heart. Its exact nature is doubtful; but it is probably only 

 a temporary formation in the oesophagus, by means of which 

 the size of the head is increased, so that it presses hard against 

 the pedicel of the egg. This pressure is the immediate cause of 

 hatching, since it forces the pedicel up like a lid. Thereupon 

 the larva, or pronymph as it is now called (c), flows swiftly and 

 easily out of the egg. 



The Pronymph is very unlike a free larva. It is closely 

 enveloped by a fine chitinous covering, the pronymphal sheath 

 (prs). This sheath not only invests the body, but closely follows 

 the form of all the limbs, which are held down by it close alongside 

 the body. The labium is only fused basally, and at present is 

 quite unsuited to perform its functions. The head and eyes are 

 moderately large. The head-vesicle still contains some fluid. 

 The pronymphal sheath ends posteriorly around the cerci in the 

 form of a sharp spine (c, sp) which usually catches against the 

 broken egg-shell, and serves as an anchor for the pronymph. 



Pierre [118] and Balfour-Browne [5] have spoken of this pro- 

 nymphal sheath as the amniotic covering. This is clearly in- 

 correct, since the sheath is a non-cellular chitinous structure. It 

 seems clear that the pronymph is really the first larval instar, and 

 its sheath just an ordinary cuticle, such as is cast at ecdysis. 



The pronymphal stage in Anax lasts only from three to twenty 

 seconds. The cephalic heart increases greatly in size. It pulsates 



