86 M.H. Rathke on the Respiratory Process in Insects. 
tected in the other abdominal segments of even the largest of 
these insects. 
§ 6. The abdominal segments of the Muscid@ also possess two 
plates of hardened epidermis. In some of these animals, espe- 
cially in the genus Musca itself, the upper plate assists to form 
a great part even of the ventral wall; in others it has but little 
to do with this. The lower plates are very considerably smaller, 
and in most of these insects resemble flat tiles. In some each 
lower plate is more elongated, and is then sometimes extremely 
narrow. But the most anterior of. these plates is always the 
largest, and the rest generally diminish in size by degrees towards 
the posterior extremity. The interspace of soft skin on each side, 
between the upper and lower plates of each segment, is narrower 
or broader, according as the upper plate occupies more or less 
of the segment. It is usually very narrow, but becomes con- 
siderably enlarged when the alimentary canal is very full, or when 
the sexual organs, especially the ovaries, have attained a great 
size. 
During quiet respiration only the lower pieces of the segments - 
move a little up and down. When respiration takes place more vio- 
lently, as, for instance, when a fly is held by the wings, the ends, or 
rather those parts of the upper plate which assist in forming the 
ventral wall, are alternately drawn in and pushed out, but at the 
same time the ventral wall, as it rises, becomes elongated, and 
afterwards, when it sinks, is again shortened; so that in the 
former case the abdomen is more straightly extended, and in 
the latter more curved downwards. When the fly is not troubled, 
the above-mentioned contraction and dilatation of the abdominal 
cavity takes place very rarely. 
The trachez have a shrub-like form, and, like the canal which 
wnites their stems on each side, are only of moderate diameter. 
In each side of the body, close to the thorax, there is a tolerably 
large air-sac. 
§ 7. In Panorpa the upper and lower plates of each abdomi- 
nal segment are of nearly equal size, and the interval on each 
side between the plates is proportionally very broad. Never- 
theless the cutis, and especially the epidermis forming this inter- 
space, are tolerably thick. Respiratory movements are very di- 
stinctly perceived in the three or four first segments of the*abdo- 
men : they consist in the alternate drawing in and pushing out of 
the skin between the plates, during which, however, the anterior 
half of the abdomen is scarcely perceptibly contracted from above 
and below. . 
§ 8. In the Lepidoptera the upper plates of the abdominal 
segments are not much larger than the lower ones. The inter- 
vening skin is tolerably. wide, as in Panorpa, but somewhat 
