660 BREATHING-APPARATUS OF LARViE OF MUSCOID FLIES, 



The plate is siirroanded by a strong band of chitin, varying in 

 thickness and in outline in different species. At the base is a 

 space or opening in the band; in Opliyra nigra, however, this is 

 closed up: and, in Pycnosoma rnjifacips and P. raripcs, it is open, 

 forming a break in the band. This space appears to carry some 

 soft structure in life, but which is destroyed in preparation. 

 Within this band are three structures radiating slightly from the 

 base; on subjection to strain, it is seen that these are slits, the 

 openings being crossed by bands of chitin, either straight or 

 anastomosing. The side-walls of the slits are xevy much 

 strengthened with chitinous blocks, and the 'ends are always 

 rounded. Thej^ are also concave from side to side. 



In some species here dealt with, there is an intermediate struc- 

 ture lying between the middle and inner slits — "inner" referring 

 to the side nearest the median line of the body. This is a 

 shallow groove, somewhat V-shaped, coming to a more or less 

 definite point towards the bottom, and rounded at the top. The 

 rounded end encloses a clear space, the margin of which shows, 

 under a low magnification, a "sun-ray" effect. This is due to 

 fine bands of chitin radiating outwardly to the band. The inner 

 ends of these chitinous bands project over the rim of the clear 

 space, and would seem to support some soft structure in life, but 

 which is destroyed by the preparation. In some species, also, a 

 spaQe resembling a blister occurs on the side of the slits; when 

 present, it is always on the outer side of the outer and middle 

 slits, and on the inner side of the inner slit. This "blister- 

 structure" also shows the "sun-ray" effect referred to in the 

 ' ' intermediate-structure. " 



The length and breadth of all the spiracular plates include the 

 width of the band. 



I am indebted to Professor Wheeler, of Harvard University, 

 U.S.A., for the method of preliminary preparation of the maggots. 

 Full-grown maggots are dropped into water heated to SO'C. A 

 few seconds in water at this temperature is sufficient to kill 

 them, and has the great advantage of stretching the maggots to 

 their full length; by killing in alcohol, the maggots generally 



