OUTLINES OF ENTOMOLOGY. . 451 



For convenience this classification may be tabulated thus : 



{ Vertebrata. f Insecta. 



j Articulata j Myriapoda. 



Animal Kingdom. ! \ Aracbnida. 



Mollusca. j Crustacea. 



Radiata. [ Annelida. 



In proceeding to separate true insects from the other members of 

 their sab-kingdom, the most obvious distinction is that they alone 

 among articulates ever possess icings. There are, however, some in- 

 sects, mostly of the worst repute, such as fleas, bed-bugs, lice and a 

 few others, that never acquire wings. We have, therefore, to seek in 

 some other organ, or set of organs, a characteristic by which we can 

 distinguish any insect from other articulates, and one which will, if 

 possible, have a peculiar development in each of the classes. Such a 

 distinction is found in the number of the legs. Thus insects, in their 

 perfect or adult state, always have six legs, neither more nor less — 

 although in some butterflies the front pair of legs, not being used, are 

 of diminished size, and are tucked up out of the way. Millepeds, as 

 the name denotes, have numerous legs, from thirty to four hundred; 

 spiders and mites have eight legs ; crustaceans from ten to fourteen legs, 

 while the true worms have no legs at all in any stage of their existence. 

 We have, therefore, only to count the legs of an articulate animal to 

 find out in which of the preceding classes it belongs.* 



The term insect, which is derived from two Latin words, in and 

 seco — cut into — in reference to the ring-jointed or insected body, was, 

 by some of the earlier writers on natural history, applied to the entire 

 group of the articulates, and the various classes were distinguished as 

 " six-legged insects," " eight-legged insects," " many-legged insects," 

 and so forth. But as the structure of these animals was more thor- 

 oughly studied they were found to be more widely separated than was 

 at first supposed. The term k ' insect" was restricted to the six-legged 

 class, and other names, more suitable, applied to the remaining classes. 

 Insects are further distinguished from other articulates in having the 

 external framework composed apparently of thirteen or fourteen rings 

 or joints, which are separated into three regions, the first joint forming 

 the head, the succeeding three the thorax, and the remaining nine or 

 ten the abdomen and its appendages. 



In the millepeds the number of rings varies from ten to two hun- 

 dred, and the head alone is distinct from the abdomen. In the spiders 

 the joints are so closely consolidated that two only can be per- 

 ceived, the first forming the combined head and thorax (cephalo thorax) 

 and the second the abdomen. In the crustaceans the number of joints 

 is variable, and in some species the division is into three regions, as in 



*Except in the case of the young of some Mites, which also have only six legs. 



