966 



INSECTA. 



longitudinal vcsxek can be discovered in the 

 muscular coat, but we bave never been able to 

 observe them. The muscular coat is most 

 distinct in the proventriculus or gizzard, the 

 ventriculus or digestive stomach, and the colon, 

 but may be traced throughout the whole of the 

 canal. The longitudinal and transverse fibres 

 are each developed to a greater or less extent in 

 different insects ; in some, more especially in 

 the larva state, as in the Lepidoptera, the longi- 

 tudinal fibres form six strong bands, arranged 

 at equal distances around the canal, and ex- 

 tended from one extremity of it to the other, 

 more or less developed in different parts of 

 their course; while in other instances, more 

 particularly in the perfect state, the circular 

 fibres are most developed, as in the Hymenop- 

 tera, in which the longitudinal fibres on the 

 stomach are scarcely observable. 



The Ttnieous, or internal coat, analogous to the 

 mucous coat in the higher animals, is divided 

 into two layers, each of which has been consi- 

 dered as a distinct structure by different anato- 

 mists. The most internal of these layers forms 

 the proper lining of the alimentary canal, and 

 is a smooth soft membrane, particularly distinct 

 in the upper part of the canal, but less so in 

 the lower. It is continuous with the lining of 

 the mouth and pharynx, and is often plaited 

 or folded longitudinally, but seldom trans- 

 versely, excepting where it covers a fold of the 

 other structures to form a valve at any part of 

 the canal. It is this membrane which is often 

 solidified at the upper part of the canal, and 

 developed into rows of strong horny teeth, as 

 in the Orthoptera and some carnivorous Co- 

 leoptera, or is covered entirely with exceedingly 

 minute ones over its whole surface, as is parti- 

 cularly the ease in Gryllus migrator ius. In 

 some instances it )s very loosely attached, and 

 forms, as it were, a soft and easily separated 

 lining to the canal, more particularly in the 

 Tentriculus, as in the Metue and some other 

 genera. The other layer of this coat, which has 

 been regarded as a distinct structure, is situ- 

 ated between the proper mucous or lining 

 membrane and the muscular coat. It is this 

 layer which is considered by Straus as the pro- 

 per skin or lining. It is usually thin, floccu- 

 lent, and frequently without indications of dis- 

 tinct texture, although it is occasionally found 

 to possess it, as shewn by Burmeister in Hydro- 

 philus. Straus has sometimes observed horny 

 prominences in it, which he considers of a 

 glandular nature. The markings in Hydrophi- 

 lus appear to be of the same description. Ram- 

 dohr mistook this layer for a layer of transuded 

 chyle ; Straus and Burmeister regard it as per- 

 fectly distinct from the mucous coat, and Pro- 

 fessor Grant in alluding to it seems to regard it 

 as a loose intermediate cellular tissue.* This 

 is our own opinion also of its nature, because 

 we have been unable to trace it as a distinct 

 layer throughout the whole of the canal. It 

 exists most distinct in the ventriculus, but we 

 have not been able to trace it in the colon, ex- 

 cepting, perhaps, in the Lepidoptera, in which 

 it appears to be what we have regarded as an 



adipose coat. The inner or true mucous layer 

 is very distinct, and in gome, as in Cerura vi- 

 nula, is covered with minute rounded glands. 



The alimentary canal is retained in its 

 position in the body partly by means of the re- 

 tractores ventriculi, which we have observed 

 most distinctly in the larvae of Diptera, as in 

 Eri&tatis tenax, as well as in Lepidoptera and 

 others ; but more especially by means of rami- 

 fications of the trachea! vessels, which pass 

 from the great longitudinal tracheae, near the 

 spiracles, and are distributed in profusion over 

 the alimentary canal throughout its whole 

 course. Burmeister says that a peritonceum, 

 such as retains the intestines in their place in 

 the higher animals, does not exist in insects. 

 This, however, as we have above shewn, is not 

 strictly the case, since a peritonceum certainly 

 exists as a coat of the alimentary canal, al- 

 though we have never been able to observe it 

 forming, as stated by Professor Grant,* "a 

 distinct thin mesentery," connecting the con- 

 volutions of the intestines with the interior of 

 the abdominal segments. 



The parts of the alimentary canal are the 

 mouth andpkarynx, the oesophagus, (Jig. 424,//,) 

 and, in the Lepidoptera, (Jig. 430, ,) Hymen- 

 optera, and Diptera, the crop, which is a dila- 

 tation of the oesophagus carried to so great an 

 extent as to form adistinct appendicular cavity ; 

 the proventriculus or gizzard (i), the ventri- 

 culus or proper digestive stomach (/f), the 

 ilium or short intestine (/), and the colon 

 (ro, n) and rectum (o). These exist in the 

 most developed form of the canal, but not in- 

 variably or to the same extent in every insect. 

 The crop is frequently absent, as are also the 

 proventriculus and the rectum, but the remain- 

 ing parts are almost constantly present. Be- 

 sides these as forming parts of the digestive 

 apparatus, there are the appendicular struc- 

 tures, consisting of the salivary glands (a), the 

 gastric (, 6, c), and the so-called biliary (p) 

 and the anal vessels (s). Of these the sup- 

 posed biliary vessels are almost constantly pre- 

 sent, and less frequently the salivary and gastric, 

 and least frequently the anal vessels, which have 

 not been observed in many species. 



Alimentary canal of the larva. The most 

 simple form of alimentary canal we have yet 

 met with in insects exists in apodal larvae 

 of parasitic Ilymenoptera, as in Ichneumon 

 Atropos y which undergoes all its changes 

 within the cavity of the abdomen, between the 

 alimentary canal and muscular structures of the 

 larva and pupa of Sphinx ligustri or Acherontia 

 AtropoB. It consists simply of an elongated 

 sac, very much dilated, and greatly resembling 

 a Florence flask, and occupies nearly the whole 

 of the interior of the body of the parasite. The 

 oesophagus is short and very distinct, and termi- 

 nates in the second segment in a well-deve- 

 loped valve formed by a duplicate of the mu- 

 cous and muscular coats. Behind this the 

 whole forms one dilated 'continuous cavity, ex- 

 tended as far as the anal segment, but com- 

 pletely imperforate and slightly intussuscepted 

 at its extremity, where, when the part is carefully 



Outlines of Comparative Anatomy, p. 349. 



Id. loc.ch. 



