M.H. Rathke on the Respiratory Process in Insects. 93 
those in which the abdomen is more or less curved downwards, 
as in the Humble Bees and Bees, the abdomen, when extended, 
is at the same time more strongly decurved. 
The shortening of the abdomen diminishes its cavity and 
causes exspiration; its elongation enlarges the cavity, and is 
connected with inspiration. Sometimes, however, in this latter 
movement, especially when the insect protrudes its sting, the 
upper plates of the segments are pushed further over the lower 
_ ones, the cavity of the abdomen is contracted nearly as much as 
it is elongated, and thus the inspiration is hindered; nay, some- 
times the upper plates may be so strongly pushed on the lower 
ones that an exspiration must be produced. The shortening of 
the abdomen (exspiration) is effected by peculiar muscles attached 
to the dorsal, ventral, and lateral walls of the abdomen. The 
ventral muscles are only of moderate size, and form cords, of 
which two always run (somewhat converging) from the anterior 
margin of each segment, nearly to the same part of the preceding 
one. The dorsal muscles take the dorsal vessel between them, 
as the ventral ones do the chain of ganglia, and are attached to 
the upper plates in the same way as these to the lower ones ; 
they are, however, rather thinner. Of the lateral muscles one 
springs from the upper and anterior angle of each lower plate, 
except that of the first segment, and passes obliquely downwards 
and forwards, gradually becoming broader, to the lower plate of 
the preceding segment; and another, from the lower and ante- 
rior angle of each upper plate in the same segments, turning 
upwards and forwards, attaches itself to the inner surface of the 
same plate in the preceding segment. Both muscles reach from 
the segments from which they spring nearly to the anterior 
margin of the other. 
The act of inspiration is effected partly by the elasticity of 
the softer skin between the segments, but still more by peculiar 
muscles, of which there are two pairs on each segment, except 
the first. In most (and probably in all) aculeate Hymenoptera 
these are much thicker than the muscles just described as being 
attached to the lateral walls and serving for exspiration. The 
inspiratory muscles are also placed in pairs on each lateral wall 
of the abdomen; they spring in each segment from the same 
spots to which the exspiratory muscles are attached, but lie 
behind these, and do not run, like them, obliquely forwards ; but, 
supposing the abdomen in the act of inspiration, the muscle 
springing from the inferior plate of each segment passes straight 
down to the hinder margin of the inferior plate of the pre- 
ceding segment; that springing from the upper plate goes 
straight up to the hinder margin of the upper plate of the pre- 
ceding segment. In the state of exspiration both muscles are 
