254 MANUAL OF ZOOLOGY. 



nised. In the imago, however, of the Cokoptera, several of 

 these primitive gangha have coalesced, so that this number is 

 considerably reduced. 



The organs of sense are the eyes and antennse. The eyes 

 in Insects are usually " compound," and are composed of a 

 number of hexagonal lenses, united together, and each supplied 

 with a separate nervous filament. Besides these, simple eyes 

 — " ocelh," or " stemmata" — are often present, or, in rare 

 cases, may be the sole organs of vision. In structure these 

 resemble the single elements of the compound eyes. In a few 

 cases the eyes are placed at the extremities of stalks or ped- 

 uncles, but in no case are these peduncles movably articulated 

 to the head, as is the case in the Podophthalmous Crustaceans. 

 The antennse are movable, jointed filaments, attached usually 

 close to the eyes, and varying much in shape in diiferent In- 

 sects. They doubtless discharge the functions of tactile organs, 

 but are probably the organ of other more recondite senses in 

 addition. 



The sexes in Insects are in different individuals, and most 

 are oviparous. 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 " meta- 

 morphosis," before attaining maturity. In some Insects, how- 

 ever, there appears to be no metamorphosis, and in some the 

 changes which take place are not so striking or so complete as 

 in others. By the absence of metamorphosis, or by the degree 

 of its completeness when present, Insects are divided into 

 sections, called respectively Ametabola, Hemimetabola, and 

 Holometabola, which, though not, perhaps, of a very high scien- 

 tific value, are nevertheless very convenient in practice. 



Section i. Avidabolic Insects. — These pass through no meta- 

 morphosis, and also, in the mature condition, are destitute of 

 wings. The young of these insects {Aptera) on escaping from 

 the ovum resemble their parents in all respects except in size ; 

 and though they may change their skins frequently, they undergo 

 no alteration before reaching the perfect condition, except that 

 they grow larger. 



Section 2. Hemimetabolic Insects. — In the insects belonging 

 to this section there is a metamorphosis consisting of three 

 stages. The young on escaping from the ovum is termed the 

 "larva;" when it reaches its second stage it is called the 

 "pupa," or "nymph;" and in its third stage, as a perfect 

 insect, it is called the "imago." In the Hemimetabola., the 

 "larva," though of course much smaller than the adult, or 

 "imago," differs from it in little else except in the absence of 



