22 AN INTRODUCTION TO ENTOMOLOGY. 



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

 covers. 



The Hemelytra. — This 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 Tegmina. — This name is given to the mesothoracic wings when they 

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

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

 tra the tegmina are also called wing-covers ; but unlike elytra they overlap each 

 other when at rest. 



The Hdlteres, Balancers, or Poisers. — With most insects that possess only the 

 first pair of wings (Diptera and the males of Coccidse) the metathorax is fur- 

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

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

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

 Coccidse 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 halteres 

 appear to aid in flight. 



The Pseudo-halteres. — 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-halteres. 



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, 

 in 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 abdomen is the third or caudal region of the body. Its 

 segments are more simple, distinct, and ring-like than those 

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

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

 didae) there are usually only three or four visible, while in many 

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

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

 pendages. But many larvae have fleshy appendages which aid in 



* The most important of the attempts to work out the homologies of the wing veins is 

 by Josef Redtenbacher. Ann. des K. K. Nat. Hofmuseums, I. (Wien, 1SS6). See Re- 

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



