65 



Tomocerus. lu tlio latter genus and some species of tlie 

 vSniintliuriclae the terminal joint is much longer than the 

 rest and is annulated. With the exception of Neelus, 

 the Sminthuridse usually have elbowed antennae. 



The thoracic segments are usually sharply defined ; 

 the prothorax is generally the smallest division of the 

 three. In the Sminthuridse the thorax is greatly com- 



Text-Pig. 1. — Abdomen of an embryo Isotoma, showing appendages 

 (after Prowazek, enlarged). v, ventral tube; hm, 

 hamula or ' catch ' ; />, furcula or ' spring.' 



pressed between the relatively large head and abdomen 

 and, furthermore, the metathorax becomes partially fused 

 with the first abdominal segment. Neelus [Megalo- 

 thorax), however, is remarkable on account of the enormous 

 development of the thorax, which greatly exceeds the 

 abdomen in length and bulk. 



The legs have no true tarsal segments, and they 

 terminate usually in two claws, an upper and a lower, 



F 



