70 THE LARVA OR NYMPH [CH. 



regularly, at about 30 beats to the minute, quickly exhausting 

 the head-vesicle, and driving blood strongly into the dorsal part 

 of the head and into the eyes. These parts swell up with amazing 

 rapidity to twice their original size. The result is a second split, 

 the pronymphal sheath bursting along the back of the head and 

 thorax, so that the young larva slips easily out (D, E). 



The moment it is free, the young larva wriggles violently, 

 drawing its legs and labium into their natural positions, ready for 

 use. The latter is now seen to be in the form of a completely 

 fused organ, the mask (D, lm), very similar to that of the full- 

 grown larva. The cephalic heart quickly collapses, and can no 

 longer be seen. In point of fact, so quickly does the hatching 

 take place, that this organ is only in action for less than a minute, 

 performing about 25 strong pulsations. In the meantime, how- 

 ever, a strong but smaller pulsating organ has appeared between 

 the rectal valves (D, rv), and appears to be pumping water into 

 the rectum. At the same time, gas can be seen travelling gradually 

 and evenly down the dorsal tracheal trunks from the region of the 

 mid-gut backwards 1 . On reaching the rectum, the gas passes 

 quickly, first into the larger rectal efferents, then into their 

 branches, and finally around each separate capillary loop, so that 

 the whole beautiful structure of the branchial basket becomes 

 quickly and evenly exposed to view. Almost immediately, 

 active respiration of the basket begins, and the larva uses the 

 ejection of the water as a means of propulsion. The pulsating 

 organ between the valves gradually subsides, but the valves con- 

 tinue to open and shut regularly. 



The newly-hatched larva has the appearance seen in fig. 24 E. 

 The head and compound eyes are very large, but no ocelli are 

 present. The antennae are only three-jointed. The labium 

 resembles that of the grown larva, and at no time bears any 

 setae in this species. The segments of the thorax are equal in 

 size, and little larger than those of the abdomen. The legs are 



1 Quite recently, I have shewn that this gas is CO 2 , and that it passes into the 

 tracheae by diffusion from the region of the mid-gut. When rectal respiration is 

 fully established, the C0 2 gradually passes out of the tracheae, and is replaced by 

 air derived from the water in the rectum. Rectal respiration is also practised 

 by Zygopterid larvae at this stage, and for the same purpose, their caudal gills 

 being practically useless at first. 



