OF CERTAIN CHALCIDIDJE AND ICHNEUMONIDjE. 91 



lopment of caeca over their whole surface, from their opening into the alimentary canal to 

 their distal terminations, which, in all insects, are csecal, and do not, in any way, anastomose 

 with any other structure ; as some have erroneously supposed them to do with the so-called 

 adipose, or splanchnic tissue. 



Thus we find that in proportion to the more or less early development of any structure 

 or organ, the function or instinct associated with that organ is more or less early evolved ; 

 and that in proportion to the completeness of a tissue, such is the degree or perfection of 

 each special function or instinct in the animal. 



Additional Note. 



Read February 15, 1853. 



The change of form and condition which the alimentary canal undergoes, after the para- 

 site has ceased to feed (fig. 8) and is assuming its imago state (fig. 10), is as remarkable as that 

 which takes place in the vegetable-feeding caterpillar, in changing to the chrysalis of the 

 future butterfly or moth. The short narrow oesophagus (e) becomes considerably elongated, 

 and instead of terminating, as in the larva, in the third or meso-thoracic segment, it is 

 extended, in the imago, through the meta-thoracic, into the abdominal region. In the 

 anterior portion of the abdomen, the fifth, sixth and seventh segments, it is then dilated 

 into a conical-shaped crop (/), which, by a constriction at its termination, and a reflexion 

 inwards of its tissues to form the cardiac valve, is separated from the true digestive cavity, 

 the stomach. This portion of the canal, the chylific ventricle (/,/), which occupied nearly 

 the whole interior of the body of the larva, is now restricted to the eighth, ninth and tenth 

 segments. It is a powerful muscular structure, of a somewhat elongated oval shape, and 

 the length of which is scarcely more than thrice its diameter. Around its termination are 

 inserted, externally, the hepatic or Malpighian organs, from twenty to thirty in number 

 (k), where, internally, by a reflexion of the tissues, is formed a second valve, the pylorus. 

 The canal then becomes narrowed into what may be regarded as duodenum and ilium, or 

 small intestine (g). Beyond this it is again dilated into a more muscular structure (h, i), 

 the colon or rectum, which is usually filled with ejecta, and terminates at the anal valve. 



The canal in the imago, as in the larva, is formed of distinct layers or tissues, a muscular, 

 a glandular, and a mucous ; and is invested, externally, by a distinct, transparent, perito- 

 neal membrane, which appears to be homologous with the peritoneal covering of the 

 viscera in the Vertebrata, and processes, or reflexions of which, in these Invertebrata, clothe 

 every internal organ, the salivary and hepatic glands, the organs of circulation and repro- 

 duction, and the adipose tissue, and tracheae ; as expressly mentioned, in regard to the 

 latter, in my article ' Insecta*.' 



The tissues of the alimentary canal are, however, much changed in condition in the 

 imago, from that in which they exist in the larva, — a change which is accompanied by 

 some alteration of function in the entire organ. In the larva, in which the canal is little 

 more than a capacious bag, the external or muscular tissue is imperfect, and consists of 



* Cyclopaedia of Anatomy and Physiology, vol. ii. part 18, p. 965 (Oct. 1839). 



