OF CERTAIN CHALCIDIDjE AND ICHNEUMONID^. 89 



Anthophora, as I ascertained many years ago, the chief portion of the digestive organ is 

 an elongated stomach, and although in this instance a short intestine and colon exist, not 

 an atom of faeces is passed, as I have many times, to my complete satisfaction, proved, 

 until the whole of the food is consumed, and the larva has attained its full size. 



IV. Development of the Alimentaey Canal and its Appendages. 



These remarks on the anatomy and development of parasitic Hymenoptera, compared 

 with their economy and instincts, lead us to inquire into the mode in which the alimentary 

 canal in Insects is formed. The first developed portions of the parietes of the body in the 

 embryo are the ventral and lateral divisions of the segments. These are produced before 

 the alimentary canal is commenced, the space between them being occupied by the yolk, 

 which supplies the means of growth to the whole. The lateral portions of the segments 

 grow from below upwards, and their free margins gradually more and more approach 

 each other, until at last they meet along the median line of the dorsal surface of the 

 body. The parts which first meet are those of the cephalic, and afterwards those of the 

 anal segments, and the junction of the remaining segments then proceeds in gradual suc- 

 cession from behind forwards, as I have witnessed in very numerous observations in the 

 embryo of Forficula. The whole of the remains of the yolk, composed entirely of masses 

 of nucleated cells, is thus gradually enclosed within the body, by the successive union 

 along the dorsal surface of the two sides of the segments, from behind forwards, the last 

 portion included being in the prothorax. The fact of the yolk entering the body at this 

 point of the thorax in the Crustacea was first pointed out by Rathke. From the remains 

 of the yolk thus included the alimentary canal is entirely formed, the external portion 

 giving origin to muscular tissue and basement membrane, and the internal, besides sup- 

 plying nutriment for the further development of the embryo, becoming organized into an 

 elaborating tissue, which for a time retains the general character of the original cell- 

 masses of the yolk, as shown in Monodontomerus. The termination of the future ali- 

 mentary canal is thus the result of a folding on itself of the first portion of the yolk included 

 by the completion of the anal and penultimate segments, and is the commencement, poste- 

 riorly, of the column of cells which, becoming perforated, constitute the future colon and 

 small intestine, and which retain the celkeform structure to so late a period in some larvae, 

 as in the instance we have seen in Monodontomerus. In this way, formed from the 

 included yolk, the digestive apparatus becomes a hollow cavity, closed, at first, at its 

 abdominal end by the approximation of the whole of its component structure, but com- 

 municating anteriorly with a canal which is formed between the parts of the future mouth, 

 and which becomes its inlet or oesophagus, the connexion of the yolk with the dorsal 

 surface of the body in the prothorax being entirely obliterated. As the growth of the 

 body proceeds, the walls of the alimentary canal become thinner, lose much of their cellse- 

 form condition, and acquire a more organized structure. The column of cells which 

 connect the great digestive cavity with the anal segment, as in Monodontomerus, are 

 gradually transformed into muscular tissue, from without inwards, and constitute the 

 future intestine, or colon and ilium. These parts being chiefly for the transit of the faeces, 

 and further elaboration of the contents of the stomach, are later developed, but acquire a 



