THE PUPAL PERIOD. 



The pupae of mosquitoes are aquatic. They are active and extremely sensitive 

 to external stimuli. With most species the natural position is at the surface of 

 the water, where they take in air by means of the two thoracic respiratory 

 trumpets. Upon the slightest alarm they go down into the water ; they progress 

 rapidly by powerful strokes of the abdomen which is furnished at the apex with 

 paddle-like organs for this purpose. While the specific gravity of the larva3 is 

 greater than the water, so that they sink when not held by the surface film, the 

 pupse are usually much lighter than the water. Vigorous strokes with the 

 abdomen are required to carry the pupa down into the water and as soon as its 

 exertions cease the pupa rises to the surface — unless, indeed, as is often the case, 

 the pupa gets under some object or holds fast by clasping with its abdomen. 

 Towards the period of eclosion the pupa grows rapidly lighter, so that, towards 

 the last, it can hardly go down. As we have shown in our discussion of the 

 ecdysis of the imago on a previous page, this increase in buoyancy is intimately 

 connected with the process of eclosion. 



The pupae of some species are much lighter than those of others. Thus the 

 pupa of Aedes atropalpus is of about the same specific gravity as the water. 

 When after going down into the water a certain distance, it ceases its efforts it 

 remains stationary, neither rising nor sinking. 



The pupffi of Mansonia are exceptional in that they do not, like the others, 

 rise to the surface for air, but attach themselves to the roots of aquatic plants. 



The thoracic breathing-tubes of the Mansonia pupa are pointed apically ; by 

 inserting them into the vascular roots of aquatic plants the pupa obtains the 

 necessary air from the plant. The process of eclosion of the imago has not been 

 observed in Mansonia. 



The pupal period of mosquitoes is brief, in many cases by far the shortest 

 period of the mosquito's life. It varies with the species and with the tempera- 

 ture. In the case of the very rapid breeders, such as Psorophora, it may be 

 less than a day. Ordinarily it is a matter of two or three days, and the period 

 is prolonged by low temperature. It appears that the large forms, like Mega'- 

 rhirnis, have the longest pupal period, even under favorable conditions as much 

 as five or six days. 



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