NO. I INSFXT THORAX — SNODGRASS IQ 



forms resemble the adults of winged insects, though it would be diffi- 

 cult to homologize individual sternal muscles in the two groups. In 

 each thoracic segment of Japyx a pair of divergent muscles goes for- 

 ward from the arms of the sternal apodeme to the posterior edge of 

 the sternal plate preceding. These muscles clearly act as retractors 

 of the anterior sclerite, their action in this capacity being made possi- 

 ble by the reversed overlapping of the adjoining sternal parts, and the 

 flexibility of the folds between the chitinous plates. The median ridge 

 of the Y-apodeme gives origin to muscles that go obliquely outward 

 to the coxae, and other coxal muscles of the sternutn are attached 

 between the arms of the apodeme. The dorsal muscles of the coxae 

 arise on the tergum. 



The sternal apodeme of the Protura and of the entognathous Thy- 

 sanura is evidently a homologous structure in both groups. It consists 

 of a median ridge in Acerentomon and Camp odea (Grassi), but is 

 forked anteriorly in Eosentamon and Japyx. This apodeme can 

 scarcely be a prototype of the furca of the Pterygota, because the 

 median base of the furca is clearly a secondary development in the 

 higher winged insects, the arms, developed from lateral sternal inva- 

 ginations, being the primitive elements of the furca. 



The thoracic musculature of Lepisma and Machilis is complicated, 

 especially so is that of Machilis, but, while a thorough study of it 

 would make a valuable contribution to our knowledge of the Thysa- 

 nura, it does not appear, from a superficial examination, that it would 

 throw any light on the evolution of the sternal musculature in the 

 Pterygota. The Thysanura do show, however, that the ventral longi- 

 tudinal muscles of the thorax have become attached to the posterior 

 margins of the sterna, which latter apparently have absorbed the ante- 

 costal ridges following, since antecostae and precostae are not present 

 in the typical position on any of the thoracic sterna, or on the first 

 abdominal sternum. 



The larvae of pterygote insects with complete metamorphosis have 

 in most cases a primary segmentation of the body, in which the attach- 

 ments of the longitudinal muscles are at the intersegmental grooves. 

 Even where segmental plates are present in the body wall, as in the 

 larvae of some beetles (fig. 25), the areas of the muscle attachments 

 may be non-chitinous, but wherever a costa is formed {ac) it is in 

 the intersegmental fold and is attached to the segmental plate follow- 

 ing it. 



In the larva of Dytiscus inarginalis, as described by Speyer (1924), 

 a pair of furcal arms arises on each sternal region of the thorax 

 between the legs, and each pair is supported on a transverse ridge, or 



