EXTERNAL ANATOMY OF INSECTS. 413 



1 . The first consists of those whose head inosculates 

 more or less in the anterior cavity of the chest; and 

 whose articulation, therefore, seems to partake in a greater 

 or less degree of the ball and socket [Enarthrosis). The 

 head, however, is often capable of being protruded from 

 this cavity. If you take into your hand any common Har- 

 palus that you may find under a stone, you will see, if 

 pressed, that it can shoot forth its head, so as to be en- 

 tirely disengaged from the prothorax : a neck of ligament 

 intervening between them a : of course this power of pro- 

 truding the head enables the animal to disengage it at its 

 will from the restriction imposed upon its motions by the 

 surrounding margin of the prothoracic cavity. To this 

 section belong all the Coleoptera, the Heteropterous He- 

 miptera (Ci?nexlj., &c), and some of the Neuropttcra [JRa- 

 pliidia, Semblis, &c). — It may be further divided into two 

 subsections — those, namely, whose head inosculates in 

 the prothorax by means of a neck : as for instance La- 

 treille's Trachelides, Apoderus, and the Staphylinidce, 

 amongst the beetles ; the Reduviadce amongst the Hete- 

 ropterous insects, and Raphidia in the Neuroptera ; and 

 those whose head inosculates in the prothorax without 

 the intervention of a neck ; as, the Petalocera, the aqua- 

 tic beetles [Dytiscus, Hydrophilus, &c), and most of the 

 genus Curculio L. in the first of these orders, the great 

 body of the Cimicidcc in the second, and Semblis, Cory- 

 dalis, &c. in the third. 



2. The second section consists of those insects whose 

 head does not inosculate in the chest, but is merely sus- 



a This was written directly after the experiment recommended in 

 the text had been tried, with the result there stated. 



