284 



METAZOAN PHYLA 



In many cases the larva bears a considerable degree of resemblance 

 to the mature insect, being hatched, however, with a relatively large 

 head and small thorax and abdomen and frequently with no more than 

 rudiments of wings. As successive molts occur the proportions of the 

 body gradually change and the wings increase in size. The stages which 

 follow the successive molts are called instars. The last molt transforms 

 the larva into an adult in which the regions of the body have acquired the 

 adult size and proportions and wings have become of full size and func- 

 tional. Thus in this type of insects there is no stage corresponding to the 

 pupa, and the metamorphosis is termed incomplete. Incomplete metamor- 

 phosis (Fig. 190) occurs, generally speaking, in the lower insects, and those 



which have it are called Hemimetabola 

 (hem i me tab' o la; G., hemi, half, and 

 metabole, change) or Heterometabola 

 (het er o me tab' o la; G., heteros, different, 

 and metabole, change), the latter because 

 metamorphosis is varied in character in 

 different types. Usually the larvae of 

 insects with incomplete metamorphosis 

 are termed nymphs, and sometimes those 

 of such of these as are aquatic, Jiaiads. 

 There is a more pronounced change when a 

 naiad, which carries on aquatic respiration, 

 becomes an air-breathing adult than when 

 the nymph of a terrestrial form changes 

 into the adult insect. 



A third group of insects, according to 

 development, is Holometabola (ho lo me- 

 tab' o la; G., holos, whole, and metabole, 

 change). These pass through a complete 

 metamorphosis, which includes both a 

 larval and a pupal stage (Fig, 191). 

 The larva of a butterfly or a moth is called a caterpillar; that 

 of a beetle, a grub; that of a fly, a maggot; and those of other 

 groups have still other names. During the larval period organs 

 may develop which are peculiar to that stage in the life history 

 of the animal. At the close of the larval period a pupa, or chrysalis, 

 is formed, which is covered with a hard shell. During the pupal stage 

 the greater part of the organs of the larva undergo degeneration, the 

 organs of the adult developing in their stead. However, the nervous 

 system is not thus "scrapped," nor are the reproductive organs. Most 

 pupae are inactive, but some are able to move about by flexion and exten- 

 sion of the body or by using the spines on the movable metameres as 

 levers. 



Fig. 189. — Young and adult of 

 Campodea staphylinus Westwood. 

 The simplest living insect, and an 

 example of Ametabola. (From 

 Kellopg, "American Insects," by the 

 courtesy of Henry Holt & Company.) 

 X 11. 



