2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 39 



during the pupal stage. Actually it simply returns to its parental adult 

 structure after having undergone during its evolutionary history a 

 metamorphosis by which it took on a form and structure suited to a 

 way of living quite different from that of its parents. The embryo, 

 from its very beginning in the egg, develops into a larva. The egg, 

 therefore, contains two distinct hereditary factors, one that first 

 produces the larva, another that later generates the parent adult. When 

 the larva does not differ too much from its parents, the adult may be 

 formed mostly by a new growth of the larval tissues ; but as the differ- 

 ence becomes more extreme, the larval tissues go into a state of dis- 

 solution and the adult is built up of embryonic cells that multiply but 

 do not become organized during the larval stage. The transformation 

 of the mosquito is intermediate between these two conditions. 



Inasmuch as the word metamorphosis means simply a "change of 

 form," we may say that the larva in its aberrant evolution has under- 

 gone a divergent metamorphosis, and that as an individual it resumes 

 the parental form by a convergent metamorphosis. 



Since the egg has the potentiality of developing into both the larval 

 and the adult form, there must be some influence that allows the larva 

 to develop first. The inhibition of adult development is effected by a 

 hormone, known as the juvenile, or status quo, hormone. When 

 the larva is mature and has served its purpose in the life of the insect, 

 this hormone ceases to be effective, and the adult development pro- 

 ceeds under the stimulus of another hormone. This at least is the 

 usual story of endocrinal regulation of insect growth and transforma- 

 tion, but, as will be seen, the mosquito does not comply fully with 

 the rules of hormone control in its growth from larva to adult. 



Before going on with anatomical descriptions of the larval, pupal, 

 and adult stages of the mosquito, a few terms should be defined as 

 they will be used. An instar is the insect between any two consecutive 

 moults. Moulting is the physiological process of separating the old 

 cuticle from a new cuticle being formed by the epidermis beneath it. 

 The new instar begins its development when the moult is completed, 

 but remains inside the old cuticle until it is fully formed. Then it 

 breaks the cuticle and comes out. The emergence of the insect is its 

 ecdysis (coming out). Moulting and ecdysis, therefore, refer to two 

 different events, and are not synonymous terms, though many en- 

 tomologists have not distinguished them as such. In life-history 

 studies the "instar" is usually regarded as the insect between ecdyses, 

 but since development begins inside the old cuticle, an instar is really 

 the insect between moults. The concealed intracuticular period of the 



