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ANATOMY AND PHYSIOLOGY 
Muscles analogous to those just noticed exist in 
insects whose pro thorax is connate, that is, intimately 
united to the succeeding segment ; but, in these the 
mesothorax being most highly developed, it is there 
that they acquire their greatest dimensions, and one 
pair is generally enlarged at the expense of another. 
Thus, the dorsal pair is most voluminous in the Hy- 
menoptera and Lepidoptera, while it is the lateral 
pair in the Diptera. 
Many of the muscles already noticed contribute, 
in a greater or less degree, to promote the act of flight, 
by contracting or dilating the walls of the thoracic 
cavity, but there are a few to which the office of 
moving the wdngs is exclusively assigned. These 
originate from the lateral parts of the sternum, and 
are attached by pointed tendons to the principal ner- 
vures of the wing. Their development is always in 
proportion to that of the wing which they are destined 
to move. If the anterior wdngs be largest, as stated 
by Burmeister, the dorsal muscle of the anterior 
wing is likewise the largest ; if the posterior wings 
are wanting, their extensor is also wanting, and 
if both are of equal size, their extensors also are 
of equal size; but, if the posterior wings are the 
largest, this is likewise the case with their extensors, 
as may be seen in the Coleoptera, while the extensor 
of the elytra in that order is very small. A small 
extensor, flexor muscles, and a series of smaller ones, 
which, when in action, cause the relaxation of the 
extensors, are the other motive instruments of the 
wings. 
