STRUCTURE 87 



rings ; but this condition is in many cases obscure ; the number 

 of these rings rarely, if ever, exceeds thirteen in addition to the 

 head and to a terminal piece that sometimes exists. Insects usually 

 change much in appearance in the course of their growth, the 

 annulose or ringed condition, being most evident in the early part 

 of the individual's life. The legs are usually elongate and 

 apparently jointed, but in the immature condition may be alto- 

 gether absent, or very short ; in the latter case the jointing is 

 obscure. The number of jointed legs is always six. 



External Structure. 



The series of rings of which the external crust or skeleton of 

 Insects is composed exhibits great modifications, not only in the 

 various kinds of Insects but even in the different parts of the 

 same individual, and at successive periods of its development ; 

 so that in the majority of mature Insects the separate rings are 

 readily distinguished only in the hind body or abdomen. The 

 total number of the visible rings, segments, somites, or arthromeres, 

 as they are variously called by different writers, is frequently 

 thirteen in addition to the head. This latter part is considered 

 to be itself composed of the elements of several rings, but mor- 

 phologists are not yet agreed as to their number, some thinking 

 this is three while others place it as high as seven ; three or four 

 being, perhaps, the figures at present most in favour, though 

 Viallanes, who has recently discussed^ the subject, considers, 

 six, the number suggested by Huxley, as the most probable. 

 Cholodkovsky is of a similar opinion. However this may be, 

 the three rings behind the head constitute the thorax, which is 

 always largely developed, though, like the head, its segmenta- 

 tion is usually very much obscured by unequal development of 

 different parts, or hj consolidation of some of them, or by both 

 of these conditions. The third great division of the body, the 

 abdomen, is also usually much modified by one or more of the 

 terminal segments being changed in form, or even entirely with- 

 drawn into the interior of the body. The existence of ten 

 segments in the hind body can, however, be very frequently actually 

 demonstrated, so that it is correct to speak of ten as the normal 



number. 



1 Ann. Sci. Nat. (7) iv. 1887, p. 111. 



