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M. H. Rathke on the Respiratory Process in Insects. 89 
approximate upper plates, passing from the anterior margin of 
each posterior plate to the imner surface of the preceding one; 
these muscles also take no part in the respiratory movements, 
but seem to shorten the dorsal and lateral walls. The above 
description. of the muscles applies only to the smaller species of 
the genus Acridium, and not to the larger ones, such as A. 
stridulum and A. migratorium. In these there is, on the outer 
surface of each process of the inferior plates, a tolerably strong, 
long, bandlike muscle, running from the apex of the process 
downwards and backwards, and attaching itself to the lower 
end of the upper plate of the same segment. If all these mus- 
cles contract, the ventral wall of the abdomen must be depressed, 
and the ventral cavity enlarged; so that these muscles must be 
regarded as inspiratory. Each segment likewise possesses a pair of 
exspiratory muscles, which are also of considerable size and form 
two flat, broad, and not very long bands, each of which springs 
from one of the outer margins of the lower plate, and, becoming 
broader, ascends upwards and outwards to attach itself to the 
lateral wall of the upper plate of the same segment. There are 
also muscles similar to those described under 2 and 3 in the 
smaller Acridia; but those of No. 1 are wanting. 
In the various species of Gryllus (or Locusta) we have,—1, mus- 
cles which agree perfectly with those described under No. 3 in 
Acridium; 2, on each side of the abdomen there is a simple 
series of small muscular bundles, which, in position and attach- 
ment resemble those described under No. 2 in the Acridia. 
These and the preceding have no particular connexion with re- 
spiration, All the other muscles now to be mentioned, on the 
contrary, are probably devoted entirely to respiration: there is 
nothing like them in Acridiwm; and they spring from a small 
and tortuous fold, which is formed by the soft skin between the 
upper and lower halves of the abdominal segments, and projects 
a little inwards towards the ventral cavity. 3. A small muscle 
passes from the above-mentioned fold, inwards, forwards, and 
downwards, to the lower part of each segment, except the first 
and the last two. 4 and 5. In each of the same segments two 
somewhat larger muscles spring on each side opposite each of 
the preceding, from the fold, pass upwards and outwards, and 
attach themselves to the end of the upper plate. In the penul- 
timate segment there is only one pair of such muscles. When 
the cavity of the abdomen is gradually extended, as especially in 
female insects by the products of the sexual organs, the soft 
space between the upper and lower plates increases considerably 
in breadth, and the muscles just described then not only become 
greatly elongated, but increase generally in size; but when 
these muscles and, simultaneously with them, those described 
