4 6 



ENTOMOLOGY 



erectile appendages of the pro thorax; and the tegulce (parapterd) of 



Lepidoptera, Diptera and Hymenoptera a pair of small sclerites at 



the bases of the front wings. 



The thorax has also several small sclerites which are not described 



here, though they are of interest to the morphologist. 



Each of the three thoracic segments bears a pair of spiracles in 



the embryo, but in most imagines there are only two pairs of thoracic 



spiracles, the suppressed pair being the prothoracic. 



The sclerites of the thorax owe their origin probably to local strains 



on the integument, brought about by the muscles of the thorax. Thus 



the primitively wingless Thysanura and 

 Collembola have no hard thoracic 

 sclerites, though certain creases about 

 the bases of the legs may be regarded 

 as incipient sutures, produced mechan- 

 ically by the movements of the legs. 

 In soft nymphs and larvae, the sclerites 

 10 do not form until the wings develop; 

 and in forms that have nearly or quite 

 lost their wings, as Pediculidae, Mallo- 

 phaga, Siphonaptera and some para- 

 sitic Diptera, the sclerites of the thorax 

 tend to disappear. Furthermore, the 

 absence of sclerites in the prothorax is 

 FIG. 59. Dorsal aspect of the tho- probably due to the lack of prothoracic 



rax of a beetle, Hydrous piceus. i, 



pronotum; 2, mesoprsescutum ; 3 , wings, not withstanding the so-called 

 mesoscutum; 4 , mesoscuteiium ; 5 , obsolete sutures of the pronotum in 



mesopostscutellum; 6, metapraescu- 



tum; 7, metascutum; 8, metascutellum; grasshoppers. 



2o R r tap S Endoskeleton.-An insect has no 



internal skeleton, strictly speaking, 



though the term endoskeleton is used in reference to certain ingrowths of 

 the external cuticula which serve as mechanical supports or as protec- 

 tions for some of the internal organs. The tentorium of the head has 

 already been referred to. In the thorax three kinds of chitinous in- 

 growths may be distinguished according to their positions: (i) phrag- 

 mas, or dorsal projections; (2) apodemes,' lateral; (3) furca, or apo- 

 physes, ventral. The phragmas (Fig. 61) are commonly three large 

 plates, pertaining to the meso- and metathorax, and serving for the 

 origin of indirect muscles of flight in Lepidoptera, Diptera, Hymenop- 

 tera and other strong-winged orders. The apodemes are comparatively 



