398 EXTERNAL ANATOMY OF INSECTS. 



(lomicile within which it retires for protection; these are 

 Molliisca. Lastly, in some animals the bone completely 

 envelopes the muscle, as with a case, which is articulated 

 externally and vertically, thus dividing the animal into a 

 number of portions, segments, or annuli ; w^hence the name 

 Annulata. 



An Insect, then, is an annulate animal ; and its characters 

 are these : — the bones are external, or perhaps, more correctly 

 speaking, the skin in which the animal is inclosed has become 

 solid, compact, and bony ; and, like the bones of vertebrate 

 animals, serves for the attachment and support of the softer 

 and muscular parts, around which it foi-ms a complete case 

 or covering, which, owing to its liability to injury, consequent 

 on its constant risk and exposure from its own activity, is 

 absolutely necessary to protect it from that loss of life which 

 must otherwise very speedily annihilate the kind. This case 

 is vertically divided into thirteen segments ; and each of these 

 segments is sometimes '' subdivided, both vertically and hori- 

 zontally, into four; thus giving sixteen osseous plates or bones 

 to every segment, or two hundred and eight to the whole 

 trunk.*" From the first of these segments arise the organs of 

 manducation, vision, and two antennce, which are the principal 

 organs of touch ; from the second, two legs ; from the third, 

 two wings and two legs ; and from the fourth, two wings an^ 

 two legs ; these ten being the organs of locomotion : these 

 organs are for the most part covered with the same osseous 

 case as the trunk, and are articulated in a similar manner. In 

 the sutures of the trunk, and also in those of the organs of 

 locomotion, the connecting skin is membranous and pliable, 

 affording freedom of motion when required; but there are 

 exceptions to this. 



The bony plates being always so formed as to meet accu- 

 rately at their margin, and to play easily, and without injury 



'' I should suppose this may be invariably the case. 



"= Mr. MacLeay asserts that fifty-two segments is the maximum number in the 

 Ckilognatha. (Anatomy of the Thorax of Winged Insects, Zool. Jour. XVIII. 

 p. 153.) In many of these, each segment very evidently consists of a dorsal, a 

 ventral, and two lateral plates or bones, which would produce the number, two 

 hundred and eight, as proposed above, and afford a striking fact in support of 

 Andouin's excellent observation, that the case of all Anmdata is formed of a fixed 

 number of parts, which may he distinct or united, but which exist in all. 



