228 THE ENTOMOLOGIST. 



The sexes may be distinguished by the structure of the 

 genitalia in the last few segments as shown in fig. 2. 



The tracheal system is completely absent in the Acerento- 

 midae, but in the Eosentomidae there are two pairs of lateral 

 stigmata, one pair on the mesothorax and the other on the 

 metathorax. According to the most recent account, that 

 of Prell [10] , the tracheae from the front spiracle supply 

 the head, thorax, and the two hind pairs of legs ; and 

 those from the posterior spiracle supply the hind pair of legs 

 and the abdomen. The front legs are without tracheae, while 

 the hind pair are supplied from both spiracles, but the two 

 systems do not communicate. Berlese's account differed slightly 

 from this, and may be consulted in his monograph [5] . 



The nervous system consists of a supra-oesophageal ganglion 

 in the head, which has a prolongation behind into the prothorax, 

 a sub-oesophageal ganglion fused with the prothoracic ganglion, 

 and a double ventral nerve-chord with ganglia in the meso- 

 and metathorax and on each of the first six abdominal segments, 

 that in the sixth segment being larger than the preceding ones. 

 There are also supplementary ganglia in the thorax at the base 

 of each leg. 



In the head and prothorax of some species of Eosentomon 

 there are several remarkable structures called "konkremente " 

 (concretions) by Eimsky-Korsakow [14]. These are small, 

 dark, almost round bodies, apparently isolated, of which there 

 are five pairs near the dorsal surface in the head, and three 

 pairs in the prothorax near the ventral surface. They are 

 arranged symmetrically, but are liable to be displaced and 

 pressed over one another in mounted specimens. They were 

 first described by Schepotieff in his Protapteron indicum [16] 

 as " innere einschlusse " ; he, however, only mentions five 

 as present in the prothorax. They were next mentioned 

 by Rimsky-Korsakow [I. c] in his species Eosentomon silvestrii, 

 and they occur in a species of the same genus, closely 

 related to and possibly identical with this last, which I have 

 taken in some numbers in England. Their function is quite 

 unknown. Rimsky-Korsakow suggests that they are the 

 accumulated secretion of some gland, while Schepotieff [16, 

 p. 341] considers them similar to Belese's "corpora allata," 

 which the latter describes on the head of Acerentomon and 

 Eosentomon. 



True eyes are lacking, but there are, one each side of the 

 head, rather nearer the front end, a pair of organs called 

 " Pseudocelli " by Berlese. Their function is not understood. 

 Becker [2, p. 398] considers them related to the " post-antennal 

 organs," which he describes in Collembola, and suggests that 

 they may have a vibratory function. 



The alimentary canal is a simple, straight tube, widening in 



