THE FRONT COXAL CAVITIES OF A BEETLE. 



15 



the front coxal cavities Midi are either "entire" when they are en- 

 closed behind by the junction of the prosternum and the epimera 

 (the episterna never reaching the coxal cavities proper), or are 

 "open behind" when a space is left protected only by a membrane. 

 They are "separate" when the prosternum extends between them, 

 or "confluent" when it is not visible between them. 



The Meso- and Metathorax. The middle or second, thoracic seg- 

 ment is called the mesothorax, and in the beetles is very closely 

 united with the third or hindmost one, the metathorax. These two 

 form the trunk or main body of the insect, and support on their un- 

 der surface the middle and hind legs and on the sides above the 

 inner wings and elytra. The upper surface of these two segments 

 is for the most part covered by the elytra and therefore invisible. 

 It consists of four pieces, only one of which, the scutellum, a small, 

 shield-shaped or triangular plate, is usually visible from above be- 

 tween the bases of the elytra. The mesothorax is much reduced in 

 size, its chief function being to support the elytra and to help keep 

 them together by means of its scutellum. 



The ventral or lower portions of these segments consist of the 

 same pieces as the prothorax, viz., the mesostemnm with its side- 

 pieces or episterna and epimera, and the met aster mini, with the 

 same. These pieces are usually distinct, except that the two of each 

 segment are often united to form a single piece. The suture which 

 separates the side-pieces of the mesosternum from those of the me- 

 tasternum is always present. The form and size of these side-pieces 

 is a character much used in classification. 



The Wings. The great majority of adult beetles possess two 

 pairs of wings. The first or outer pair, known as the elytra (sin- 

 gular elytron), or wing covers, are present, except in a few females 

 of the family Lampyridae, are horn-like in texture and vary greatly 

 in shape and sculpture. They are attached to the mesothorax and 

 usually cover the upper surface of the abdomen, but in many genera 



[2—23402] 



a \i c. 



Fig. 4a. p, indicates the prosternum; cc, the coxal cavities; es, the thoracic side- 

 pieces. In a the coxal cavities are widely open behind and confluent; in b they are 

 narrowly open behind and separate; in c they are closed behind and separate. 



(After Wickham.) 



