THE THORAX AND ITS APPENDAGES. 13 



together. These pieces are more distinct iu some insects than in others, 

 but they can always be distinguished by impressed lines upon the sur- 

 face called sutures. The three pieces of the thorax are distinguished as 

 the fore-thorax, the middle-thorax, and the hind thorax ; or, iu scientific 

 language, tho, ]iro-thorax^ the meso-th 'vax, and the meta-thorax. In the 

 Coleoptera the pro-thorax is very large, and forms the large upper part, 

 or shield, to which we usually give the general name of thorax. In this 

 order of insects, the meta thorax is invisible above, and the only part 

 of the meso- thorax seen from above is the triangular j^iece between the 

 bases of the elytra, called the scutellum. 



In many insects {Hymenoptera and Lepidoptera) the pro-thorax is 

 mrch reduced in size, and forms only a narrow rim, which is usually 

 called the collar. 



The under side of the thorax is called the sternum or breast plate. 

 Each of the three divisions of the thorax has its sternum, designated 

 respectively as the pro-, meso- and meta-steriium. In many insects, 

 and especially the Coleoptera, each section of the sternum is divided 

 by sutures into a middle piece or sternum proper, and a side piece, called 

 the episternum. These parts will be described more particularly in 

 treating of the Coleoptera. 



The appendcujes of the thorax are the organs of motion, namely, the 

 winys and the legs. 



The Wings. — The great majority of insects have four wings. The 

 anterior pair are attached to the upper part of the meso-thorax, and the 

 posterior jjair to the meta-thorax. 



The wings are thin, membraneous, transparent organs, in some cases 

 folded when at rest, and supported by ribs or veins ruuniug across them. 

 These veins are found to correspond in their number and complexity to 

 the rank of the insect in the scale, and from the ease with which they 

 can be seen, they furnish admirable characters for the purposes of class- 

 ification. In some insects, such as the grass-hoppers, the fore- wings are 

 thicker and less transparent than the hinder pair, and have nearly the 

 consistency of parchment ; and in one large order of insects, the Cole- 

 optera or beetles, the fore-wings become converted into the hard opaque 

 pieces, known as the elytra or wing-cases. The elytra take no part in 

 flight, but serve only to cover and protect the hinder or true wings, 

 which are folded under them when at rest. 



In one large order, the insects have but two wings, and are named 

 from this character Biptera, or two-winged insects. In these insects the 

 place of the hind-wings is supplied by a pair of little knobbed append- 

 ages called halteres or poisers. 



There are a few exceptional cases of two-winged insects in some of 

 the other orders— for example, some of the smaller Day-flies {liJphemera') 

 iu the order of JSTeuroptera, and the males of the Bark-lice {Coccidw) in 

 the order of Homoptera. 



