580 EXTERNAL ANATOMY OF INSECTS. 
tion, which probably keeps them from dislocation in re- 
pose, and, by the sudden disengagement of these points 
from the cavities, assists the animal in jumping *. 
2. Prophragma®. ‘'Thisis a piece usually almost ver- 
tical, but in Zlater horizontal; of a substance between 
membrane and cartilage, descending anteriorly from the 
dorsolum, and forming the first partition of the chest of 
the mesothorax; it is generally much shorter than the 
mesophragm, Though very visible in Coleoptera and 
the Heteropterous Hemiptera, in the other Orders it is 
less easily detected, and is sometimes obsolete. It may 
be observed here, that in the Hymenoptera, at least in 
the wasp, the hive-bee, the humble-bee, and the Dipte- 
ra mostly, the interior of the upper-side of the alitrunk, 
instead of two, seems at first to be divided into four cham- 
bers, formed by septula : but as these ridges merely mark 
out the internal limits of the dorsolum, scutellum, postdor- 
solum, and metapnystega, the last but one of these being 
usually less distinct, they seem not analogous to the three 
partitions of the alitrunk in other Orders; so that in 
these the mesophragm at least seems to have no repre- 
sentative, and the prophragm and metaphragm include 
between them only oneample chamber. In the Dzptera, 
wherever there is an external depression or suture there 
is a corresponding internal 7¢dge or seam, so that the 
parts seem more distinctly marked out on the inside 
than on the owtszde of the crust. 
3. Mesophragma’. ‘This piece also, which forms the 
middle partition of the upper part of the cavity of the 
alitrunk, dividing it into two chambers, is most conspi- 
* Vor. II. p. 314, » Prate XXII. Fie. 8, 11. H. 
© [pide Bre. 9; lees: 
