102 THORAX CHAP. 



The parapteron of Audouin does not appear to be really a 

 distinct portion of the pleuron ; in the case of Dytiscus it is 

 apparently merely a thickening of an edge. Auclouin supposed 

 this part to be specially connected with the wing-articulation, 

 and the term has been subsequently used by other writers in 

 connexion with several little pieces that exist in the pleural 

 region of winged Insects. 



The prothorax is even more subject to variation in its 

 development than the other divisions of the thorax are. In the 

 Hymenoptera the prosternurn is disconnected from the pronotum 

 and is capable, together with the first pair of legs, of movement 

 independent of its corresponding dorsal part, the pronotum, 

 which in this Order is always more or less completely united 

 with the meso-thorax ; in the Diptera the rule is that the three 

 thoracic segments are closely consolidated into one mass. In 

 the majority of Insects the prothorax is comparatively free, that 

 is to say, it is not so closely united with the other two thoracic 

 segments as they are with one another. The three thoracic 

 rings are seen in a comparatively uniform state of development 

 in a great number of larvae ; also in the adult stages of some 

 Aptera, and among winged insects in some Neuroptera such as the 

 Ernbiidae, Termitidae, and Perlidae. In Lepidoptera the pro- 

 notum bears a pair of erectile processes called patagia ; though 

 frequently of moderately large size, they escape observation, being- 

 covered with scales and usually closely adpressed to the sides of 

 the pronotum. 



The two great divisions of the body the mesothorax and 

 the metathorax are usually very intimately combined in winged 

 Insects, and even when the prothorax is free, as in Coleoptera, 

 these posterior two thoracic rings are very greatly amalgamated. 

 In the higher forms of the Order just mentioned the meso- 

 sternum and mesopleuron become changed in direction, and form 

 as it were a diaphragm closing the front of the metasternum. 

 The ineso- and rneta-thorax frequently each bear a pair of wings. 



We have described briefly and figured (Fig. 55) the sclerites 

 of the mesothorax, and those of the metathorax correspond fairly 

 well with them. In addition to the sclerites usually described 

 as constituting these two thoracic divisions, there are some small 

 pieces at the bases of the wings. Jurine discriminated and 

 named no less than seven of these at the base of the anterior 



