IN THE LARV^ OF ANISOPTERID DRAGONFLIES. 155 



that the arrangement o£ the trachese is very similar to that ah'eady seen in 

 Aiistrocesclma. There is, however, a more complicated branching o£ the 

 trachese, and the finer branches run very close up to the border o£ the leaf, 

 giving off numerous fine capillaries which arch along the free border of the 

 leaf, and then turn over downwards to join other branches. These latter 

 unite up, finally, to enter the main trachea of the gill at a different level. 

 Thus the whole folia, but especially its free distal border, is richly supplied 

 with capillaries. 



Differentiation of the Posterior Portion of the Gill-basket. — In the gill- 

 basket of ^lEschna there is a very large and broad anterior portion followed 

 by a much smaller and narrower posterior portion. On everting the basket 

 by a longitudinal cut, it will be seen at once that there is a difference in 

 the arrangement of the folise in the anterior and posterior portions. In 

 the anterior portion the folise are closely crowded together, the spaces 

 between succeeding folise in a hemibranch being very short. In the pos- 

 terior portion, however, the number of folise is very small ; they are exceed- 

 ingly well developed, and seem to lie more freely in the rectal cavity. Also 

 they are separated from one another by much wider intervals. Text-fig. 6 

 shows the arrangement of the folise in the anterior part, while text-fig. 7 

 shows some folise from the posterior part. 



Presence of Aborted Main Longitudinal Folds. — In order to show these 

 clearly, I fixed and cleared the whole abdomen of a well-grown larva of 

 ^schna brevistyla. While in cedar-oil, this was cut into thick transverse 

 sections (2 mm.) by means of a sharp razor. On examining these sections 

 under a low power, the six main longitudinal folds developed in the Simplex 

 System of the young larva (see p. 181) could be clearly made out, separating 

 the double rows of hemibranchs of the Duplex System of the more mature 

 larva. They are devoid of dark pigmentation, and carry no trachese. They 

 may be followed right back to the posterior end of the gill-basket, where 

 they pass into the six corresponding " rectal glands." The small posterior 

 terminations of the gill-hemibranchs may be actually seen in the same 

 section as the "rectal glands,^' at the posterior end of the gill-basket, which 

 in this genus passes, without any sphincterial separation, into the anal 

 portion of the rectum (text-fig. 8). 



The study of this genus, then, makes it quite certain that the main longi- 

 tudinal gill-folds of the Simplex System are actually homologous with the 

 so-called " rectal glands," with which they agree both in number and 

 position. 



Number of Capillaries in the Gill-basket. 



In y^schna brevistyla there are 16 folise in a hemibranch (rarely 17 or 18). 

 In each folia there are 8 or more branches of the main tracheal stalk, and 

 each branch may be credited with a minimum of 50 capillary loops. Hence 



