350 CHARACTERS OF INVERTEBRATE ANIMALS 



antennae, maxillary palps, labial palps, and cerci and those of 

 vision, as represented by the large compound eyes. There is 

 good reason to believe that the antennae are organs of smell as 

 well as of touch, and it is probable that some of the mouth parts, 

 perhaps the second maxillae, have to do with taste. It may be 

 further remarked that great difficulty attaches to the interpretation 

 of many of the sense organs possessed by animals (see p. 264), 

 and Insects, among others, are undoubtedly endowed with special 

 senses of which it is difficult, if not impossible, for us to form 

 any idea, our only definite standards being our own sense organs, 

 which are those of much specialized land animals. In a subsequent 

 chapter this interesting subject will be more fully discussed. 



Life-history. An insect such as a Butterfly or Moth goes 

 through widely different stages in its life - history. Hatching 

 from the egg as a caterpillar, it grows to a considerable size, 

 and then becomes a motionless chrysalis, from which the adult 

 insect later on escapes, its body having been formed by a series 

 of revolutionary changes from the substance of the chrysalis 

 body. This life-history, familiar to all who have kept silkworms, 

 and furnishing a useful metaphor to moralists of all centuries, is 

 one which exhibits complete change of form, or metamorphosis, 

 to use the technical expression. The life-history of the Frog 

 (see p. 254) is an example of the same kind of thing, the 

 essence of which consists in considerable modification in form and 

 structure after the animal has been hatched out and has started 

 a free independent existence. Insects differ very much among 

 themselves as to the amount of metamorphosis, and the Cockroach 

 exemplifies an order in which it is so slight as to be practically 

 absent. The eggs are laid in horny capsules, each containing 

 sixteen, and the just-hatched Cockroach closely resembles the 

 adult, from which it chiefly differs in its smaller size and the 

 absence of wings. As growth proceeds, the firm exoskeleton is 

 not increased in size to suit the enlarging body as in a Mollusc, 

 but is bodily thrown off or "moulted", a very common phenomenon 

 among Arthropods. After some seven moults the adult size is 

 attained. 



Classification of Insects. Considering that some 250,000 

 distinct species have been described, and that at least ten times 

 this number are believed to exist, it will be realized that the 

 classification of insects is likely to present some difficulties, espe- 



