STRUCTURES USED IN CLASSIFICATION. 17 



much in relative length and thickness and this variation is 

 extensively used in classification. 



The Thorax and its Appendages. — The middle region of 

 the body of a bug or other insect is called the thorax. To study 

 its parts aright, the wings and legs attached to it should be 

 removed, when it will be seen to consist of three rings or seg- 

 ments. These are known as the prothorax, mesothorax and meta- 

 thorax. Within these rings are located the muscles for moving 

 the wings and legs, as well as some of the digestive organs. 



The prothorax of the bug usually has its entire dorsal sur- 

 face, and sides in great part, covered by a large piece known 

 as the pronotum. This varies much in shape and size in the 

 different families of Heteroptera. Its upper surface is called 

 the disk, and its deflexed sides or flanks the propleura. The outer 

 edges separating the propleura from the disk are the lateral 

 margins. These are sometimes carinate or raised and sharp 

 and are often sinuate, toothed or crenate. The disk is often 

 divided crosswise by a groove or impressed line, thus forming 

 two divisions or lobes. The front one of these sometimes has 

 a second impressed line or stricture just behind its apex, the 

 area in front of this being known as the collar. On the upper 

 surface of the front lobe, or front portion of the disk, are 

 often two slightly elevated, usually smooth areas known as 

 callosities or colli. The angles where the lateral margins of 

 the disk meet the hind margin of the pronotum are known 

 as the lateral or humeral angles, but are sometimes called the 

 humeri. The under or ventral side of the prothorax is a nar- 

 row, somewhat movable piece called the prosternum. Near its 

 outer ends are shallow sockets in which are attached the coxae 

 of the front pair of legs. Between the bases of these coxae is 

 a triangular piece known as the prosternal xyphus. 



The mesothorax and metathorax, the second and third segments 

 of the thorax, are, in the bug, rather firmly united with the 

 basal abdominal segment of the abdomen to form a firm walled 

 box. Lying immediately above the mesothorax is the meso- 

 seutum. This is usually covered by the hind lobe of pronotum 

 and base of outer wings, though sometimes it is partly ex- 

 posed as in figure 1. To the mesothorax are attached the elytra. 

 or outer pair of wings and the second or middle pair of legs. 

 To the metathorax are joined the inner wings and the third or 

 hind pair of legs. The under or ventral portion of these seg- 



