180 
Dipterous Larvae 
at the bases of these rods, but these “ nuclei ” are merely the optical 
effect produced by the intersection of rods, or processes from them, at 
another focal plane. In every case the slits are very much longer than 
they are broad ; are usually all about the same length (although in 
some instances the second slit is somewhat longer than the other two) ; 
and the extremities are always rounded off. 
In most larvae, at the inner side of the circumference of the plate, 
or, as is the case generally, situated on the ring, there is what has been 
previously referred to as the “ button,” a term used by Mr Banks. 
This structure, so far as I have been able to determine, is the place of 
attachment of muscles. It is not however invariably present in the 
slit class of stigmata, but its exact position, when it does occur, varies 
in different species, and is, when taken in conjunction with others, one 
of the characters on which the differentiation of the larvae can be based. 
The thickened periphery of the stigmal plate, which has been called 
the “ ring,” is sometimes incomplete, and when this is so it is at the 
region where the “ button ” ought to be situated that this break often 
occurs. An example of such a condition is to be found in the Sarco- 
phagidae. The significance of this condition I have not been able to 
discover, but it is a valuable character in the work of diagnosis, since 
only two examples of this arrangement have yet been found, namely in 
the Sarcophagidae and in Ckrysomyia. 
The Ptyehotreme-type of Stigmata. 
Examples of this type are somewhat less numerous than those of 
the preceding type, and while for the most part the arrangement is 
to a great extent similar to those just described, yet in many details they 
are entirely unlike. The “ ring ” is usually very well marked, but some¬ 
times the chitin of the plate is so dense that the inner margin of the 
“ ring ” is difficult to distinguish. Within the “ ring ” however there is 
arranged a chain, convoluted to a varying degree, and with remarkably 
different configuration according to the different species. It may be 
either in the form of an unbroken chain, or broken into segments, as 
in Plate XV, fig. 8; but in this class are included only those forms of 
stigmata which have a distinct convoluted chain, of one kind or another, 
within the outer chitinous ring ; and to decide on this point as to whether 
a particular type does, or does not, belong to this class has never been 
found a matter about which there could be nmch doubt. The typical 
examples of the class here illustrated are those of the house-fly (Musca 
