90 M.H. Rathke on the Respiratory Process in Insects. 
under No. 3 are contracted, the cavity of the abdomen is dimi- 
nished from above and below. 6. A series of seven pretty strong 
muscular bundles, each of which always lies close behind the 
other, and has its axis directed from before backwards, covers 
the greater part of the above-mentioned fold on each side of the 
body. Each bundle corresponds with an abdominal segment ; 
but whether it is connected with respiration is uncertain. 7. 
Lastly, wherever two segments are contiguous, a tolerably strong 
muscle passes transversely from the right to the left fold, and 
conceals on each side the contiguous extremities of each pair of 
muscles described under No. 6. When these muscles contract, 
the folds of skin to which they are attached are drawn a little 
inwards, and the cavity of the abdomen is narrowed laterally in 
the neighbourhood of these folds. These latter muscles also are 
greatly extended when, towards the close of summer, the gene- 
rative organs are much enlarged and the abdomen is filled with 
an accumulation of fat, but they are not thereby weakened in 
their action. Between these muscles and the ventral wall there 
is a considerable space, in which the chain of ganglia and four 
very wide air-tubes are situated. 
The trachez occur in greater number in the Gryilli ; and those 
which belong to the body-wall, with the exception of their final 
ramifications, are of considerable diameter, forming elongated 
and sausage-like tubes. Notwithstanding their width, they do 
not collapse when cut through, from their having strong and 
very elastic spiral fibres in them. They also become dilated again 
after being compressed. There are four or five pairs of vesicular 
dilatations immediately above the ventral wall of the abdomen, 
and a pair of much larger ones in the thorax above and behind 
the first pair of legs. In the Acridia the tracheal stems on 
the walls of the abdomen are fewer, but partly also much wider 
than in the Gry/li, and, from their great diameter, their collaps- 
ing when cut, and their not expanding again completely when 
compressed, they form a sort of transition to the air-sacs. Their 
spiral fibres are comparatively thin and but slightly elastic. 
§ 12. It is remarkable that Acheta campestris, although ex- 
ternally less nearly related than the Acridia to Gryllus verruci- 
vorus and others of its genus, nevertheless resembles the Grylii 
much more than the Acridia in its respiratory apparatus. 
This insect is essentially distinguished from the Locuste 
(Grylli) in the structure of the skin of the abdomen, only by the 
lower plates of the abdominal segments being proportionally 
larger and especially longer, and by the first segment possessing a 
lower plate. Parallel to these plates, but at a tolerable distance 
from them, there are, as in Gryllus, several transversely stretched 
muscular bundles, passing from the soft parts of one lateral wall 
