NOTES ON THE MYIASIS-PRODUCING DIPTERA. 253 



the medical officer was emphatic in his statement that they were actually passed out, 

 and had not just been deposited on the faeces. In any case, Chrysomyia megacephala 

 certainly does not breed in human excrement, so far as I know. It would be worth 

 while carrying out some more experiments in this direction to see whether the eggs of 

 Calliphokinae can hatch out in the intestine, and the larvae reach maturity and be 

 passed out in a living condition. 



Lastly, it is necessary to refer to the larvae of Famiia canicularis from the urinary 

 tract. There is at present no satisfactory explanation as to how such larvae can reach 

 the human bladder. The usual explanation that they pass down the urethra when 

 very small, seems to me to be highly improbable, at least in the case of the male 

 urethra, for it will be remembered that the larva of Fannia, even when small, is not 

 entirel}^ smooth, and would cause very considerable irritation. The presence of such 

 larvae in this organ is at present a mystery, and the subject requires to be 

 re-investigated. 



Identification of the Larvae of Myiasis-producing Oiptera. 



It will be remembered that the larvae of the higher Diptera are soft cylindrical 

 maggots, usually of a greyish white to yellowish white colour. The head end is 

 pointed and the body gradually increases in diameter backwards, the posterior end 

 being truncated, with a concave surface looking upwards and backwards. Such a 

 larva is divided into segments, the number of which is at present a matter of dispute, 

 and it is not possible to homologise them with those of the adult. The head contains 

 a characteristic skeleton consisting of a number of sclerites, spoken of as the cephalo- 

 pharynx. The mouth is armed with a pair of mandibular hooks, and the anterior 

 spiracles open at the sides of the apparent third segment. The posterior stigmata are 

 placed on the concave surface of the apparent eighth abdominal segment. Banks, 

 MacGregor and Metcalf have drawn attention to the value of the posterior stigmata 

 in determining the larvae of the higher Diptera. Metcalf in particular says that, 

 " So superior are the characteristics drawn from this part of the larva, that I regard 

 most descriptions which omit consideration of them as practically worthless. For 

 in most cases the general features of shape, colour, etc., may be found to apply almost 

 equally well to other species. The most available, absolutely diagnostic characters 

 are to be found on the posterior stigmata." In my own studies of the larvae of Diptera 

 I have found that the characters of the posterior stigmata, cephalopharynx and 

 anterior spiracles, taken together afford the best and only reliable means of accurately 

 determining the larva of any species of Diptera, and more particularly of those 

 of the higher Diptera. In making a preparation of the posterior stigmata of a Muscid 

 larva for purposes of identiiication, I always make it a rule to include the cephalo- 

 pharynx and anterior spiracles as well. I will now shortly describe these structures 

 in the various types of larvae of the myiasis-producing Diptera, and point out how 

 their characters can be utilised in determining any particular larva. 



Posterior stigmata. In the third-stage larvae of all the higher Diptera the posterior 

 stigmata consist of a pair of chitinous plates, which, when cleared in caustic potash, 

 are seen to be surrounded by a ring of chitin, usually spoken of as the peritreme. 

 Situated within this ring there are three or more openings which lead into the tracheae. 

 Each opening is guarded by a number of delicate chitinous rods, giving the 

 appearance of a grating ; these rods form a network across the inner side of the 

 openings. Plate 5, fig. 1 , represents a vertical section through one of the stigmatic 

 plates of the third-stage larva of Pollenia stygia. The deeply shaded portions 

 represent the chitinous parts of the plate, and the lighter-shaded parts the epidermis. 

 The three slits open into a large cavity, to the margins of which the trachea is attached ; 

 Lowne termed this cavity the vestibule. Just inside each slit there is a series of 

 branched yellow chitinous rods, known as the grating, and lining the whole cavity 

 a delicate network of epidermal fibres. The grating is evidently of use in preventing 



(4183) T 



