590 ARTHROPODA. 



The typical number of somites in the abdomen of the Insecta is 

 ten or eleven, and this number can sometimes be recognised in the 

 Orthoptera and some other forms. In the Hymenoptera and Lepi- 

 doptera not more than nine or ten can be recognised, and in many 

 cases even fewer can be made out. The abdominal somites are 

 usually more or less freely movable upon one another, and never 

 carry locomotive limbs. The extremity of the abdomen is, however, 

 commonly furnished with appendages, which are connected with the 

 generative function, and not infrequently serve as offensive and de- 

 fensive weapons. Of this nature are the ovipositors of Ichneumons 

 and other insects, and the sting of Bees and Wasps. In the Earwig 

 (Forficula) these caudal appendages form a pair of forceps ; whilst 

 in many insects they are in the form of bristles, by which powerful 

 leaps can be effected, as is seen in the Spring-tails (Podurtz). 



Generally speaking, the young insect is very different in external 

 characters from the adult, and it requires to pass through a series 

 of changes, which constitute the " metamorphosis," before attaining 

 maturity. In some Insects, however, there appears to be no meta- 

 morphosis, and in some the changes which take place are not so 

 striking or so complete as in others. By the absence of metamor- 

 phosis, or by the degree of its completeness when present, Insects 

 are divided into sections, called respectively Ametabola, Hemimeta- 

 bola, and Ifolometabola, which, though not, perhaps, of a very high 

 scientific value, are nevertheless very convenient in practice. 



The " Ametabolic " Insects are those which have no proper 

 " metamorphosis," the young stages of the Insect resembling the 

 adult in all essential points except in size. This absence of a meta- 

 morphosis is only seen in Insects which are destitute of wings in 

 the adult condition, and which are therefore often spoken of as the 

 Aptera. In the so-called " Hemimetabolic " Insects, where an " in- 

 complete" metamorphosis exists, the young insect is at first very 

 different from the adult, and in the process of conversion into the 

 latter it undergoes changes of form, while it at the same time 

 remains capable of locomotion and of nourishing itself. In its first 

 condition, after emergence from the egg, it is known asa " larva," 

 and at this stage it presents no traces of wings. In its second con- 

 dition — the stage of the "pupa" — it possesses rudimentary wings, 

 but is still active and feeds. In its third stage, as the perfect insect 

 or " imago," the wings are fully developed, and the insect acquires 

 the power of flight. Lastly, the Insects which undergo a " complete " 

 metamorphosis, and which are therefore said to be " holometabolic," 

 pass through the same series of changes as those observed in the 

 case of the Hemimetabolic forms, but the larva, pupa, and imago 

 differ from one another more widely than is the case in the latter, 

 and the insect in the " pupa " stage is quiescent and does not feed. 



