OUTLINES OF ENTOMOLOGY. 



INSEICTS IN GENERAL. 



Insects constitute the most numerous and diversified class of the 

 second grand division of the animal kingdom, designated by the term 

 Articulata, and so called because their bodies and limbs are com- 

 posed of many pieces, connected together by movable joints or articula- 

 tions. 



Insects as a class, and in the widest meaning of the word, comprise 

 three divisions, or sub-classes, commonly known as Spiders, Insects and 

 Millipedes. They may be distinguished by the following characters : 



1st. Sub-class : Arachnida, including Spiders, Scorpions and Acari, 

 or Mites. Body divided into two parts, the head and thorax being 

 united in one ; legs eight in number ; without wings. 



2d. Sub-class : Insecta, or Insects proper. Body divided into three 

 parts, the head, the thorax, and the abdomen ; legs six ; furnished with 

 wings, in the perfect or imago state. 



3d. Sub-class : Myriapoda, commonly called Millipedes or Centi- 

 pedes. Body divided into many parts or segments, varying from ten 

 to two hundred ; legs numerous ; usually either one or two pairs of legs 

 to each segment of the body ; never have wings. 



The exceptions to these characters are very few. In the Arachnida, 

 some of the most minute Acari have but six legs. 



Insects proper are always six-legged in their last or perfect state, and 

 they also generally have six true legs in their larva state; but some 

 larvcV, have no legs, and the larvae of the Lepidoptera, commonly called 

 caterpillars, have, in addition to their six true legs, several pairs of 

 false legs, or pro-legs, which assist in locomotion. 



There are a few exceptional cases in which insects are destitute of 

 wings. The Fleas (Pulices), the Lice (Pediculi), and the little family of 

 insects known as Spring-tails (Thysanoura), never have wings. In 

 some rare instances the females are wingless, whilst the males have 

 wings. This is the case with some species of the Lightning-beetles 

 (Lampyridic), and with the Canker-worm moth, and the Tussock-moth, 

 and a few other species amongst the Lepidoptera. Similarly excep- 

 tional cases are also found in other orders of insects. 



The present work will treat only of Insects proper. 



