vii APTERA 193 



iii the case of the exsertile vesicles of the Thysanura. The pro- 

 cesses in Smyntliurus bear glandular structures at their ex- 

 tremities. It has been suggested that the ventral tube of Collem- 

 bola is the homologue of a pair of ventral appendages. The term 

 Collophore has been applied to it somewhat prematurely, seeing 

 the doubt that still exists as to its function. 



Some of the Collembola possess a very curious structure 

 called the prostemmatic or ante-ocular organ ; its nature and 

 function have been very inadequately investigated. The ocular 

 organs of the Collembola consist, when they are present, of 

 isolated ocelli placed at the sides of the head like the corre- 

 sponding organs of caterpillars ; the prostemmate is placed 

 slightly in front of the group of ocelli, and has a concentric 

 arrangement of its parts, reminding one somewhat of the com- 

 pound eyes of the higher Insects. This structure is represented 

 in Fig. 100, B, C ; it is said by Sir John Lubbock to be present 

 in some of the Lipuridae that have no ocelli, and he therefore 

 prefers to speak of it as the " post-antemial " organ. 



A very characteristic feature in the Collembola is the slight 

 development of the tracheal system. Although writers are far 

 from being in accord as to details, it seems that stigmata and 

 tracheae are usually absent. In Smyntliurus there are, however, 

 according to Lubbock, whose statement is confirmed by Meinert 

 and Tullberg, a pair of stigmata situate on the head below the 

 antennae, and from these there extends a tracheal system through- 

 out the body. Such a position for stigmata is almost, if not 

 quite unique in Insects ; Grassi, however, seems to have found 

 something of the kind existing in the embryo of the bee. 



At present only a small number of species of the Order 

 Aptera are known ; Lubbock recognised about sixty British 

 species, and Finot sixty - five as found in France. The j^orth 

 American forms have not received so much attention as the 

 European, and the Aptera of other countries, though they are 

 probably everywhere fairly numerous, are scarcely known at all. A 

 few have been described from the Indo-Malayan region and some 

 from Chili, and the writer has seen species from the West Indian 

 and Sandwich Islands. All the exotic forms as yet detected are 

 very similar to those of Europe. 



The Thysanura are probably not very numerous in species, and 

 appear to be in general intolerant of cold. With the Collembola 

 VOL. v o 



