254 MAJOR W. S. PATTON. 



the entry of foreign particles into the vestibule, and the epidermal fibrous network 

 probably aids in retaining the air in the vestibule. The tracheae are lined externally 

 by characteristic peritracheal cells. Situated on the inner and lower side of each plate, 

 and usually within the chitinous ring, there is a small raised area, spoken of by Banks 

 as the " button." In a vertical section through this structure it will be noted that it 

 is a fine channel into the vestibule and is lined by a layer of peritracheal cells; so far as 

 I can ascertain, it is an accessory opening, but I am unable to say at present whether 

 it is functional or not. It is certainly not a point of attachment for muscles, as it 

 is within the vestibule. This short description will enable the reader to understand 

 this wonderful bit of apparatus. All the types which I have sectioned are of similar 

 structure, no matter whether the tracheal slits be straight, as in the Calliphorinae, 

 curled as in Mitsca, or multiple as in Oestrus and allied genera, so that I can see little 

 use in splitting them up into various types according to the shape of the slits. The 

 important point to be ascertained is whether the straight or the curled slit is the more 

 primitive, and this can only be ascertained by a comparative study of the stigmatic 

 openings of a large number of the first stage larvae of the higher Diptera. An oval 

 opening, such as is present on the anterior spiracles of most Dipterous larvae, seems 

 to be the most primitive type, and from such an opening the various slits seen to-day 

 in the posterior stigmata of the higher Diptera may have originated, but this is at 

 present mere conjecture. 



Turning now to the various kinds of slits, leading into the tracheal vestibule of 

 the posterior stigmatic openings, we note the following tj^pes : — (1) Curled slits, 

 (2) straight slits, and (3) round or oval openings. 



1. Curled slits. This type of opening is characteristic of the larva of Musca and 

 allied genera, the three openings occupying an excentric position on the plate, and 

 the ends of the upper and lower appearing to coalesce with those of the middle slit. 

 The button area is well within the chitinous ring, which, in this case, is often 

 D-shaped and broad. In Plate v, figs. 2, 3, 4 and 5, are illustrated the posterior stigmata 

 of Musca domestica, Musca nehulo, Musca hiimilis and Philaematomyia crassirostris, 

 and without going into details it will be seen that, though very closely similar 

 to each other, it is possible to note small distinctions sufficient for determining the 

 species. But I would like to point out that the determination is considerably 

 facilitated by comparing the structure of the cephalopharynx and anterior spiracles 

 of each species at the same time. Recently I was able to determine the larva of 

 Musca [Philaematomyia) crassirostris sent me from a case of intestinal myiasis by 

 comparing the posterior stigmata with those of Indian species of Musca. Curled 

 slits are also seen in the posterior stigmata of the larvae of the Gasterophilinae, 

 especially in the larvae of the species of Gyrostigma. Here, however, the grating is 

 simpler than in the larvae of Musca. 



2. Straight slits. This type of slit is characteristic of the opening of the posterior 

 stigmata of the larvae of the Calliphorinae, Sarcophagidae and some Oestridae, 

 such as the larvae of Cohboldia. In the larvae of the Calliphorinae the posterior 

 stigmata are of two kinds, one typically seen in the larvae of Calliphora and Lucilia, 

 in which there is no break in the chitinous ring and the button is enclosed by it ; the 

 other in Chrysomyia and Cochliomyia, in which the ring of chitin is so thin on the inner 

 and lower angle of the plate as to give the appearance of abreak, the button lying in 

 what is but a thin layer of chitin. This type is illustrated in Plate vi, figs. 9, 10 

 and 11, which represent the posterior stigmata of Calliphora erythrocephala, Lucilia 

 sericata and Lucilia argyricepliala, all drawn to the same scale. Further, it should 

 be noted that in each of these there are characteristic breaks in the chitin, between 

 and close to the slits, appearing as round or oval spaces ; these clear spaces are 

 extremely constant in the same larva. This close similarity between the posterior 

 stigmata of Calliphora and Lucilia makes it difficult to separate the larvae of Calli- 

 phora erythrocephala from those of Lucilia sericata. Veterinary officers may find 



