22 
POPULAR SCIENCE REVIEW. 
abdomen will consist of tbe sixth and following ones. Of these, 
seven are visible on the dorsal surface, and two at least, if not 
three, are concerned in the formation of the reproductive 
organs, making fourteen or fifteen segments in the insect, 
counting the head as one. The dorsal and ventral plates are 
well developed, the lateral ones scarcely at all. In order to 
allow room for the respiratory movements, the dorsal plates are 
soft and yielding, sufficient protection being afforded to the 
upper surface by the strong covering of the elytra. 
In our review of the thorax, in order to give a more con- 
nected view of the several plates composing the same, we have 
omitted the subject of the wings and legs ; and these important 
organs must now claim some of our attention. In the Worms, 
whose habits of life require little facility for locomotion, every 
segment takes part in the act of progression, as is also the case 
with the larvae of insects themselves, at least those which 
undergo a complete metamorphosis ; but in the perfect insect, 
where much greater facility of motion is required, we find the 
locomotive appendages confined to the thorax, where they re- 
ceive their highest degree' 5 of finish. In the Annelids each 
segment bears two pairs of lateral appendages, or rudimentary 
limbs, the superior being dorsal and the inferior ventral. The 
wings of insects would seem to represent the former, and the 
legs the latter. The prothorax of insects is not ordinarily pro- 
vided with superior appendages, but traces of such a provision 
exist in the pupae of the Diptera, where they are adapted as 
breathing organs, the anterior extremity of the great tracheal 
trunks being carried into them; they disappear, however, in 
the imago. The development of the wings may be most ad- 
vantageously studied in the common Cockroach, where they are 
seen to be gradually differentiated from the lateral portions of 
the dorsal plates of the wing-bearing segments. They are in 
principle sac-like processes of the integument, into which pass 
branches of the tracheal system. They thus always exhibit a 
double integument. The nervures are folds of the double 
integument, forming tubes along which the tracheae run. The 
elytra, beside their modification of form, differ from the 
wings chiefly in the greater amount of chitinous deposit they 
contain, and their consequently increased solidity. This 
thickening, however, takes place in the outer integument only, 
the inner one lying next the body of the insect being soft and 
membranous. Between the two, in the Stag Beetle, there is a 
vesicular layer (Pl. II. fig. 5), formed by the ramifications of 
the tracheae, ending in little bladders somewhat similar to those 
which occur in the body of the insect. This doubtless serves as 
a soft air-cushion to press upon the delicate wings beneath. 
The wings in this insect — as in most of the Coleoptera — are 
