520 The Animal Kingdom 



cial senses which are difficult to assess ; for instance, some seem to pos- 

 sess a humidity sense or a temperature sense. 



Experimental evidence has shown that many activities such as molt- 

 ing, metamorphosis, and color dispersion are controlled by endocrine 

 glands. Chief of the glands is the corpus allatum located near the supra- 

 esophageal ganglia. Other masses of secretory cells have been demon- 

 strated in different parts of the insect body, but their function is not 

 entirely clear. 



The sexes are separate, and fertilization is internal. Often there 

 is marked sexual dimorphism. Among a few forms, parthenogenesis 

 is common. In the aphids, this is the chief method of reproduction 

 during the summer. The eggs contain a large amount of yolk, and 

 often are bizarrely decorated with sculpturings of various sorts. Char- 

 acteristically there are two coverings to the eggs, an outer tough shell, 

 the chorion, and a delicate membrane, the vitelline membrane, sur- 

 rounding the cytoplasm. Minute pores, micropyles, are present in the 

 chorion. In the early stages, only the nucleus divides ; these are scat- 

 tered throughout the yolk. Gradually they coalesce, and the various 

 germ layers are formed. The postembryonic development varies greatly 

 among the members of the different orders. In some there is virtually 

 no change in size from the immature form to the adult ; these orders 

 are grouped as the ametabola. In a second group, there is a partial 

 change in size and relationships of body structures with the nymphal 

 stage closely resembling the adult. These orders are known as the 

 hemimetabola. In the final group of orders, there is a complete change 

 of appearance from the young or larval stages to the adult. These are 

 the holometabola. In their life history, four distinct periods of de- 

 velopment are recognized in this last grovip : the egg, larva, pupa, and 

 adult. This represents a complete metamorphosis in contrast to the 

 partial metamorphosis of the hemimetabola and the lack of metamorpho- 

 sis in the ametabola. 



These life histories are subject to many modifications. Among some 

 flies and beetles, the larvae themselves are able to reproduce. This is 

 known as paedogenesis. Some of the parasitic hymenoptera have eggs, 

 each of which is able to develop into thousands of individuals by the 

 process termed polyembryony. 



At times during the life of many insects there are periods when 

 the activity of the animal is suspended temporarily. This may occur 

 at any time and is known as the diapause. In immature stages, there 



