412 EXTERNAL ANATOMY OF INSECTS. 
1. The frst 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 (Enarthro- 
sis). The head, however, is often capable of being pro- 
truded from this cavity. If you take into your hand any 
common ground beetle that you may find under a stone, 
you will see, if pressed, that it can shoot forth its head, 
so as to be entirely disengaged from the prothorax: a 
neck of ligament intervening between them*: of course 
this power of protruding the head enables the animal to 
disengage it at its will from the restriction imposed upon 
its motions by the surrounding margin of the protho- 
racic cavity. To this section belong all the Coleoptera, 
the Heteropterous Hemiptera, and some of the Neuro- 
ptera (Raphidia, Semblis, &e.).—It may be further di- 
vided into ¢wo subsections—those, namely, whose head 
inosculates in the prothorax by means of a neck: as for 
instance Latreille’s ‘Trachelides, Apoderus, and the Sta- 
phylinide, amongst the beetles ; the Reduviade amongst 
the Heteropterous insects, and Raphidia in the Neuro- 
ptera ; and those whose head inosculates in the protho- 
rax without the intervention of a neck; as, the Petalo- 
cera, the aquatic beetles (Dytzscus, Hydrophilus, &c.), 
and most of the Rhyncophora, in the first of these Orders, 
the great body of the Heteropterous Hemiptera in the 
second, and Szalis, Corydalis, &c. in the third. 
2. The second section consists of those insects whose 
head does not inosculate in the chest, but is merely sus- 
* This was written directly after the experiment recommended in 
the text had been tried, with the result there stated. 
