84H 



INSECT 



preference to the others, because they are most fully 

 developed, and most complicated in their structure. 

 The pieces composing one of these segments are di- 

 yided by M. Audouin, who devoted a great deal of 

 attention to this branch of the subject, into three dis- 

 tinct kinds, dorsal, lateral, and ventral. Mr. Mac 

 Leay, however, considers them only as dorsal and 

 Tentntl, uniting the lateral with the ventral. The 

 ventral portion consists of a simple piece, of which the 

 form is very varied ; this is named the sternum. 

 Attached internally to this piece, at its posterior mar- 

 gin, is another single piece, variable in its form, but 

 generally resembling the letter Y, and appearing ne- 

 cessary for the support and protection of the nervous 

 system. This piece, as being attached to the head, 

 thorax, or abdomen, is respectively termed the ento- 

 cephalus, entothorax, and entogaster. The lateral 

 pieces are more complicated, each being composed of 

 two pieces ; the one, anterior, articulating with the 

 sternum, and directed upwards, constituting the epi- 

 sternum, behind which the other is situated, and which 

 is in connexion with the base of the leg, and is called 

 the epimeron. The upper surface of the thoracic seg- 

 ment is also composed of a number of distinct pieces, 

 which are respectively named praescutum, scutum, 

 sen tell urn, and postscutellum, and which successively 

 follow each other. Such is supposed, theoretically, 

 to be the structure of each segment of the body of 

 the insect ; and the fixed number of parts thus disco- 

 vered, is supposed to exist in all annulose animals, 

 sometimes being distinct as when the segment has 

 reached its maximum of development, but still more 

 frequently more or less rudimental, or soldered to ad- 

 joining pieces, as required by the variation of deve- 

 lopment. Some pieces may even entirely disappear, 

 and the segment may appear to consist rather of a 

 single piece, or, on the contrary, of two or even a 

 greater number of segments united together. Modi- 

 fications of this kind are traceable not only in different 

 insects, and in the different segments of the same 

 insect, but even in any single determinate segment, 

 when examined in the various states of the same indi- 

 vidual. The comparative examination of these seg- 

 ments of the insect skeleton has singularly simplified 

 the anatomical study of these animals, placing it upon 

 a firm basis, and more rigorously compelling the pre- 

 cise determination of organs which had previously 

 been either entirely overlooked, or had only been 

 regarded without reference to the analogous forma- 

 tions of other insects ; and hence to any organ which 

 happened to be singularly developed, a name was 

 given without the least reference to the comparative 

 structure of other insects, or the same comparative 

 parts in the other segments of the same individual. 



It would carry us too widely into the field of theo- 

 retical inquiry, were we to show that the head, and 

 various abdominal segments, were organised in a 

 manner strictly analogous to those of the thorax ; 

 neither could they be done effectually without a re- 

 ference to the typical structure of annulose animals in 

 general a vast and difficult subject of investigation. 

 We will, therefore, now confine our attention to . 



A The head, or the seat of the organs of sensation. 



The head, or first segment of the body, is united 

 to the anterior part of the thorax by a distinct ar- 

 ticulation, and may be considered as a kind of solid 

 corneous case or box, having an opening in the centre 

 of its anterior part, where the mouth is placed. It is 

 of a triangular or oval shape, with the narrowed part 



advanced in front, the middle of the opposite side, or 

 base, being occasionally prolonged into a neck. But 

 there are numerous modifications of this form, and 

 the head is often buried as far as the eyes in the 

 prothoracic cavity. On minutely examining the skull, 

 it will be found that it consists of several regions. 

 These are, the epicranium or skull-cap, composing 

 the greater part of the head, of which it chiefly occu- 

 pies the upper and posterior part. This 5? bounded 

 in front by the clypeus (c), or shield of the mouth, 

 which generally lies above the parts of the mouth (m], 

 and the under side of the head is covered by the jugu- 

 lum, or throat. At the sides of the epicranium are 

 situated the large compound eyes (), between which, 

 when present, the ocelli (o), or simple eyes, are 

 placed, and the antennae (a) are inserted nearer to 

 the mouth. But all these parts will require a more 

 precise examination. 



Figs. 53, Head of Dyticus ; 54, d'tto Oxycheila; 55, ditto Helio- 

 cantharus ; 56, ditto Acanthothorax. 



The head is generally of a corneous, or coriaceous 

 texture, although at times it is membranous or vesi- 

 culose ; and its form is modified from oval, or glo- 

 bose, to longitudinal, heart-shaped, compressed, cor- 

 nuted, rostrated, &c. The parts of the epicranium, 

 and its frontal part, the clypeus, have been variously 

 regarded and named by different authors. Thus 

 Kirby and Spence divide the upper surface of the 

 head into the nose (rhinanum), hind nose (postnasns), 

 forehead, crown, occiput, cheeks, and temples, and 

 the under surface into the lora and jugulum ; and it 

 is not to be denied that, in the numerous modifica- 

 tions which occur in the form of the heads of insects, 

 there are to be found portions developed to an ex- 

 tent, and to which, from their analogy with the parts 

 of the head of the higher animals, it may seem ser- 

 viceable to give analogous names ; but it is impossible 

 to limit the extent of these parts ; aud therefore a 

 general definition could only be applied to each ; as, 

 that the forehead is that part of the head which is 

 between the eyes, and when the head begins to 

 assume a flattened form ; and the cheeks those parts 

 which lie at the sides of the head between the eyes 

 and the mouth. In many insects the head is united 

 to the thorax by a membranous tube or neck, consist- 

 ing, as in the Diptera and Hymenoptera, in which the 

 motions of the head are very tree, of the attenuated 

 fore part of the thorax. In some beetles, however, 

 the term neck is applied to a.narrowed portion of the 

 skull, which forms a kind of rotule, playing in the 

 opening of the thorax. 



The clypeus (c) of Fabricius, or the nose of Kirby 

 and Spence, or the epistomis of Latreille, is an im- 

 portant part of the head, as occasionally, from its de- 

 velopment, and the consequent want of development 

 of the true upper lip (labrum), it takes the functions 

 of the latter, and serves, as the name imports, and as 

 it was at first employed by Linnaeus in the lamelli- 



