ENTOMOLOGY 



small and may be absent. Each pleuron consists chiefly of two sclerites 

 (pleurites, Fig. 58), separated from each other by a more or less oblique 

 suture. The anterior of these two, which joins the sternum, is termed 

 the episternum; the other, the epimeron. The former is divided into two 

 sclerites in Odonata and both are so divided in Neuroptera. 



The sternum, though usually a single plate, is in some instances divided 

 into halves, as in the cockroach, or even into five sclerites (Forficulidae) 

 To these should be added the patagia of Lepidoptera a pair of erec- 

 tile appendages of the prothorax; and the paraptera, or tegula, of Lepi- 

 doptera and Hymenoptera a pair of small 

 sclerites at the bases of the front wings. 



Each thoracic segment bears a pair of 

 spiracles in the embryo and in some adults 

 as well (Campodea, Heteroptera), but in 

 most imagines there are only two pairs of 

 thoracic spiracles, the suppressed pair 

 being usually the prothoracic. 



The sclerites of the thorax owe their 

 origin probably to local strains on the in- 

 tegument, brought about by the muscles 

 of the thorax. Thus the primitively wing- 

 less Thysanura and Collembola have no 

 hard thoracic sclerites, though certain 

 creases about the bases of the legs may 

 be regarded as incipient sutures, produced 

 mechanically by the movements of the 

 legs. In soft nymphs and larvae, the 

 sclerites do not form until the wings 

 develop; and in forms that have nearly or quite lost their wings, as 

 Pediculidae, Mallophaga, Siphonaptera and some parasitic Diptera, the 

 sclerites of the thorax tend to disappear. Furthermore, the absence of 

 sclerites in the prothorax is probably due to the lack of prothoracic 

 wings, notwithstanding the so-called obsolete sutures of the pronotum 

 in grasshoppers. 



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 protections 

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

 been referred to. In the thorax three kinds of chitinous ingrowths may 

 be distinguished according to their positions: (i) phragmas, or dorsal 



FIG. 57. Dorsal aspect of the 

 thorax of a beetle, Hydrous piceus. 

 I, pronotum; 2, mesopraescutum; j, 

 mesoscutum; 4, mesoscutellum; 5, 

 mesopostscutellum; 6, metaprae- 

 scutum; 7, metascutum; 8, meta- 

 scutellum; 9, metapostscutellum. 

 After NEWPORT. 



