THE THORAX AND ITS APPENDAGES. 13 



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

 but they can always be distinguished by imi)ressed lines upon the sur- 

 face calU'd sutures. The three pieces of the thorax are distin,<;uished as 

 the fore-thorax, the middle-thorax, and the hind thorax; or, in scientitic 

 hmguage, the j^t'o-thorajc, the meso-tk rax, and tlie meta-thorax. In the 

 Colcoptera the pro-thorax is very large, and forms the large upper ])art, 

 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-tliorax seen from above is the triangular piece between the 

 bases of the elytra, called the scntcUum. 



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

 mrcli 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-, mesa- and meta-sternum. 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 epistcrnum. These parts will be described more particularly in 

 treating of the Coleoptera. » 



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

 icings and the legs. 



The Wi7igs. — The great majority of insects have four wings. The 

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

 posterior pair to the meta-thorax. 



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

 folded when at rest, and sui)ported by ribs or veins running 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- 

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

 thicker and less transi)areut than the hinder pair, and have nearly the 

 consistency of parchnjent; 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 

 tiight, but serve only to cover and i)rotect 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 Diptcra, or two-winged insect;?. 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-tlies [Ephemera') 

 in the order of JS^europtera, and the males of the Bark-hce [(Joccidie) in 

 the order of Uomoptera. 



