232 NATURAL HISTORY. 



constituted two divisions of a single group (Annulosa), the leading idea in the foundation of which 

 was the division of the body in this way into a series of segments, the greater part of which might 

 be almost exact repetitions of each other. In the Arthropods, or jointed-1 imbed animals, however, 

 we find, superadded to the simple Annulose type of structure, a greater or less number of jointed 

 limbs serving the animal for progression either in walking or swimming. 



Of the Arthropoda thus characterised the creatures known as Insects constitute the highest or 

 most highly specialised type. They may be roughly defined as Arthropods in which the body is 

 distinctly divided into three parts, called the head, the* thorax, and the abdomen, furnished with 

 three pairs of jointed limbs attached to the second division of the body, and breathing air by 

 means of fine tubular organs which ramify in all parts of the body. Besides the legs the thorax 

 generally bears one or two pairs of wings ; the head has only a single pair of the organs known 

 as antenna; and the segments of the abdomen, or third division, have no limbs attached to them. 

 These statements apply to the insect in its adult or perfect state ; at earlier stages of its existence, 

 as will be seen hereafter, it may present very different characters. 



The segmented structure is most plainly shown in the abdomen (Fig. 1), and we will therefore 

 commence by describing its general characters. There are usually no more than nine segments in this 

 part of the body, and of these some ai-e often suppressed or greatly reduced in size and concealed by 

 others, so as to make the abdomen appear to consist of fewer than the theoretical full number of 

 rings. In certain insects, on the other hand, there are, beyond the true ninth segment, certain parts 

 which are regarded as representing two more segments, thus giving eleven as the total number of 

 such parts that may exist in the abdomen of an insect (Fig. 2). The segments of the abdomen, as 



already stated, bear no jointed organs of the 



2^4-56789 10 11 



^ T - L^ nature of limbs, but at the extremity of this 



part of the body we not unfrequently find 

 certain appendages (jointed tails, ovipositors, 

 claw-like pieces, &c.). The abdomen may be 

 attached to the next division of the body (the 

 thorax) by its whole breadth, or the segment 

 or segments towards the base may be more 



Fig. 2.-siDK VIEW * ^Ticcs, SHOWING or less narrowed> so that ^ Uncomm0 nly 



o, ovipositor. the actual union is effected by a very thin 



stalk. The segments themselves are composed 



of two half rings placed on the upper and lower surfaces (dorsal and ventral plates), and these may be 

 firmly united to each other at the sides, or attached by a flexible membrane similar to that by which 

 the successive segments are joined. In some cases where the wings form a protective covering for 

 the abdomen (as in Beetles), the upper surface of the abdomen shows no horny plates, but is covered 

 with a soft flexible skin. 



The thorax (Fig. 1) consists always of three segments, but, although in many insects this structure 

 is as plainly recognisable as in the abdomen, it is very often masked, or even partly concealed, either 

 by the close union of the segments, or by the shifting of the position of the parts of which the segments 

 are composed, for the thoracic segments are more complex than those forming the abdomen. Thus 

 in a perfect thoracic segment we can distinguish a dorsal plate (notuni), and opposite to it a sternum 

 (or " breast-bone"), and uniting these on each side two other pieces (pleurae) placed one behind the 

 other, of which the foremost is called the episternum, and the hinder one the epimerum. All these lower 

 pieces generally take part in forming the socket for the attachment of the limb ; they are united by 

 sutures, but frequently so amalgamated together that the whole thoracic segment seems to form a 

 complete ring. The three segments of the thorax are indicated in descriptions by distinctive names ; 

 the foremost is the protJiorax (Fig. 1), the second the mesothorax, and the third the metathorax. 

 In like manner the two principal pieces of which each successive segment is composed are 

 distinguished as the pronotum, mesonotum and metanotum, and prostermim, mesosternum, and 

 metasternum. In many cases the mesonotum exhibits in its middle a small raised plate, called the 

 scutellum (Fig. 1), which is well seen in most Beetles ; and in some insects a similar elevation is 

 presented by the metanotum, the post-scutellum. 



