ANTHOMYIA CANICULARIS. 255 



sixths. Its greatest diameter does not exceed one third of a line, and its 

 least is that of a mere thread. The whole structure appears, to be that of 

 an animal adapted to live on fluid nourishment. 



" The principal external openings to the tracheae appear to be two aper- 

 tures situated on the dorsum of the last segment of the body, and which, 

 constitute the last pair of the series of dorsal eminences formerly noticed. 

 These apertures correspond with the very remarkable and conspicuous pair 

 of organs occupying a similar situation in the last segment of the oestrus 

 of the sheep, and which are also the external openings of the respiratory 

 apparatus in. that insect. 



" None of the insects were alive when they came into my possession, 

 and they were placed in spirit of wine for the purpose of preserving them 

 previously to their being dissected. Several days after my attention had 

 thus been directed to the subject, I happened to observe at the bottom of 

 a jug of New River water a small living object, which appeared very much 

 to resemble the larvae which I had recently been examining ; and upon 

 placing this under the microscope I found the resemblance to be complete, 

 except that the animal was only about two thirds the size of the former. 

 It was deficient also in the pinna? upon the lateral spines, which were 

 simple, but the mandibular apparatus was perfectly formed. On making 

 further search two other individuals of the same species were found in 

 different stages of growth ; the smallest, however, not exceeding one third 

 of a line in length, though still possessing some of the characters of the 

 larger ones. One of these specimens was very lively, moving freely at the 

 bottom of the water, and frequently protruding and retracting its pro- 

 boscis, by which it dragged itself along. 



" This fact is a matter of some interest, as furnishing a clue to the 

 source of these parasites, since it is evident that the larvae can pass along 

 the water-pipes which supply the metropolis, and may thus be swallowed 

 in the water used for food : and in the present case the larvae, or the ova, 

 must have traversed a distance of at least a mile. At the same time it is 

 evident that this cannot always afford an explanation of their mode of 

 entrance into the body, because in the case of the clergyman at Cambridge, 

 it is expressly stated that ' he never drank water unmixed, but generally 

 beer, tea,' and the like ; at the same time the water used for these bever- 

 ages was entirely supplied from a pond on a stiff clay. If, therefore, the 

 ova found entrance with the fluid aliments, they must have withstood the 

 action of heat, as in making the beer, tea, &c. ; while, on the other hand, 

 it is difficult to suppose that they passed in with the solid food, because 

 the larvae are evidently aquatic. Perhaps the most inexplicable part of 

 the case is, the fact of their occurrence in such immense numbers. In the 

 Cambridge case several quarts were passed in a few months, and in the 

 instance which I have just recorded they were described as coming away 

 by handfuls. It is extremely difficult to account for this fact, because a 

 number of larvae, or their ova, must have been swallowed equal to those 

 which were evacuated, since they could not multiply by generation in the 

 alimentary canal, they being in the larva state, and having, as the dis- 

 section showed, and as is well known in the case of larvae, the generative 

 organs undeveloped ; indeed, no trace of generative organs was visible : 

 while it is difficult to suppose that the parent animal could have been 

 accidently swallowed, and its ova, previously impregnated, have become 



