252 GRAMMAR OF ENTOMOLOGY. 



on young animals, inhabiting the hair or down, 

 particularly of pigeons in a domesticated state. 

 Pupa perfectly quiescent, with little appearance of 

 the perfect insect ; changes in the same situations. 



820. Imago with antennae many-jointed; usually 

 concealed in cavities in the skull, but capable of 

 being erected at pleasure ; mouth as in the blood- 

 sucking Diptera ; eyes simple ; wings none ; seg- 

 ments of the body thirteen, equally developed ; 

 hind legs formed for leaping. The flea is an 

 inhabitant of all countries, feeding on the blood of 

 animals. Pulex. 



821. It must also be observed, that several 

 genera are not referred to any of the foregoing 

 orders : this arises from a doubt occurring as to 

 their correct situation ; a point not always to be 

 decided without a knowledge of the insect in all 

 its stages. 



822. The other tribe of animals with six 

 jointed legs (Aptera), contains comparatively but 

 a very small number of species; they are dis- 

 tinguished from the Tetraptera by undergoing no 

 change in external appearance after leaving the 

 egg, and are invariably without the slightest ap- 

 pearance of wings. The Aptera are divided into 

 two classes. 



823. In the first class ( Thysanura) the mouth 

 has the usual parts fully developed ; a labrum, a 

 labium, two mandibles and two maxillae ; the 

 mandibles have a perfect masticatory power, and 



