THE CLASS INSECTA, AND THE ORDER DIPTERA 29 



doll), and it may be quite inert or it may (as in the case of 

 the mosquito-pupa) be able to move freely. Sometimes the 

 pupa is merely enclosed in a thin pellicle ; in other cases it 

 secretes a chitinous exudation, which becomes hard and dark 

 and binds all its appendages together, and it is then known 

 as a chrysalis Cx/ouo-aXA£? = the metallic golden skin of the 

 butterfly's pupa) ; or the pupa may He in its cast-off larval 

 skin, which becomes firm and dark and forms a protective 

 chamber or puparium ; or the pupa may be enclosed in a 

 silky cocoon spun by the larva just before it became quiescent. 

 The dormant stage ends suddenly when the development is 

 complete, the insect splitting its pupal skin and breaking out 

 of its puparium or cocoon as a sexually mature imago, to fly 

 abroad as soon as it has shaken out its wings. In this 

 "complete" metamorphosis the larval stage is the stage of 

 feeding and growth: very often it is the longest of the 

 three stages ; in some cases it lasts for years. The imago 

 stage is the stage of reproduction, and is commonly, though 

 by no means always, of short duration : in some extreme 

 cases it endures no longer than a day. Though the phases 

 of " complete " metamorphosis appear to be abrupt, they are 

 not always really so ; the internal rudiments even of the 

 wings can often be made out in the larva by simple dis- 

 section. 



The adult insect, or imago, is typically aerial ; but there 

 are many beetles and bugs and even some Orthoptera and 

 Hymenoptera that live in water, and there is one genus of 

 bugs [^Halobates) that is found only on the surface of the 

 open sea. Again, there are many insects, aerial in the adult 

 stage, the larvae and pupse of which are true aquatic animals. 

 Many insects are parasitic on, or in, animals and plants, 

 and the parasitism may be restricted to the larval stage, or 

 to the adult stage, or may involve the whole term of life. Of 

 the external parasites of animals some are merely predaceous, 

 while others are true settled parasites. 



Insects are arranged in orders according to the features 

 presented collectively by the wings, mouth-parts, and meta- 

 morphosis ; and the orders have been grouped by Dr Sharp 

 in four series. 



I. Apterygota (a privative, and 7rTe/3iiywToy = winged). — 



