NO. I 



INSECT THORAX — SNODGRASS 



17 



ever, will be described in the discussion of the structure of a wing- 

 bearing segment (page 45). The pterygote protergum shows various 

 modifications of form, but none of its characters is to be homologized 

 with the special structures of the mesotergum or metatergum, a fact 

 indicating that the rudimentary wing lobes of the prothorax of the 

 Palseodictyoptera (fig. 19) were never developed into movable ap- 

 pendages in later insects. 



THE STERNUM 



The most important thing to be noted in a study of the sterna of 

 the insect thorax is the fact that the ventral longitudinal muscles. 



,lPh Sp2 



Fig. 7. — Pleuro-sternal parts of thorax of a cicada (Tibicina scplendeciiii). 



A, ventral view of thorax of mature nymph, legs removed, showing the 

 pleural parts continuous around inner sides of coxal cavities, suggesting that 

 each pleuron represents a basal, or subcoxal, segment (Sex) of a leg (see 

 figs. 15 B, 16 D). B, ventral view of mesothorax and metathorax of adult, show- 

 ing pleural folds on inner sides of coxal cavities persisting as chitinous ridges 

 on edges of sterna. 



except in the Protura, extend between posterior parts of the sternal 

 sclerites, and are never attached as they are in the abdomen, or as are 

 the dorsal muscles of both thorax and abdomen, to anterior ridges of 

 the segmental plates. The ventral musculature is greatly reduced in 

 the thorax of all adult pterygote insects, but in holonietabolous larvae 

 ( Coleoptera, Lepidoi^tera, Diptera) the sternal muscles of the thorax 

 are developed in proportion to the rest of the body musculature, and 

 include large latero-ventral bands of fibers continuous with the muscle 

 bands of the abdomen. In the Protura, according to Berlese (1910), 

 the longitudinal musculature is complicated in a manner characteristic 

 of primitive forms, and presents features suggestive of the muscula- 

 ture of larvs of holometabolous insects. The ventral muscles, for 

 example, consist of two latero-ventral bands of fibers continuous 



