582 EXTERNAL ANATOMY OF INSECTS. 



and the two last to the metathorax ; each forming a kind of 

 chamber of the under-side of each segment of the thorax. 



1 . Phragma. The phragm, or septum of the protho- 

 rax, is most conspicuous in the mole-cricket {Gryllotal- 

 pa), in which it is a hairy ligament attached to the inside 

 of the upper and lateral margins of the base of that part: 

 inclining inwards, it forms the cavity which receives 

 the mesothorax. It is not, however, without a representa- 

 tive in many Coleoptera, though in these it is less striking, 

 from its being smaller and taking; a horizontal direction. 

 In Elater, by means of some prominent points received 

 by corresponding cavities of the vertical part of the base 

 of the elytrum, it forms a kind of ginglymous articula- 

 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 3 . 



2. Propkragma b . This is a piece usually almost ver- 

 tical, but in Elater 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 too, 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- 



» Vol II. p. 318. ■» Plate XXII. Fig. 8, 11. h . 



