AN INTRODUCTION TO ENTOMOLOGY. 

 in a straight line on the middle of the back. The elytra are also called wing- 



Tkt Hemtlytra.1\\\* term is applied to the first pair of wings when they 

 are horny or leathery at the base and membranous at the apex, as in the Hete- 

 roptera. 



The TtgmiiM. This name is given t. the mesothoracic wini;s when they 

 are of an uniform leathery or parchment-lite texture, and are furnished with 

 veins. Of this form are the mesothoracic wings of Orthopteru. Like the ely- 

 tra the tegmina are also call< / but unlike elytra they overlap each 



other when at rest. 



The Halteres, Balancer* > s. With most insects that possess only the 



first pair of wings (Diptera and the males of Coccidac) the imtatlmrux is fur- 

 d with a pair of appendages which are believed to be the homologues of 

 the second pair of wings. These are called halteres, balancers, or poisers. The 

 halteres are usually club-shaped or thread-like, terminating in a knob. In the 

 Coccidae each of the halteres is usually furnished with a bristle which is hooked 

 and fits into a pocket on the wing of the same side. In this case the halt 

 appear to aid in flu 



The Pseudo-haltercs, The insects belonging to the family Stylopidae have 

 only the metathoracic wings developed. The mesothoracic wings are repre- 

 sented by a pair of slender club-shaped appendages, which are termed 

 pseudo-halter es. 



Venation. The number and situation of the veins of the wing afford charac- 

 ters which are much used in descriptive works. The variations presented by 

 these characters are very great. And unfortunately no one has published an 

 exhaustive work on the subject.* Much has been written upon it; but most of 

 the writers have confined themselves to a single order or even family of in- 

 sects. The result is that many systems exist ; and frequently homologous veins 

 bear different names in different groups of insects. It is necessary, therefore, 

 .n the study of any order of insects to learn the system or systems that have 

 been established for that order. The more important of these are explained in 

 the discussions of the orders in the following chapters of this work. 



The Abdomen. 



The abdC'{Utn is the third or caudal region of the body. Its 

 segments are more simple, distinct, and rin^-like than tlin.M- 

 of the other regions. The number of segments of which it ap- 

 pears to be composed varies greatly. In the Cuckoo-flics (Chrysi- 

 did;u) there are usually only three or four visible , while in many 

 other insects nine appear. Except in the lowest order of ins. 

 (Thysanura) the abdomen of the adult bears no locomotive ap- 

 pendages. Hut many larvae have fleshy appendages which aid in 



* Th most important of the attempts to work out the homologics of the wing vc; 

 by Josef Rcdtenbachcr. Ann. des K. K. Nat. 1 1 of museums, I. (Wicn, 1886). See Re- 

 view in Am. Nat. vol. xxi. p. 932. 



