NO. 8 ANATOMICAL LIFE OF THE MOSQUITO SNODGRASS 43 



It is truly remarkable that the same kind of structural adaptation 

 for the same purpose has occurred twice in the life of the same indi- 

 vidual, affecting two different organs. Furthermore, with the acquisi- 

 tion of a new structure designed for a new use, the insect must be 

 twice endowed with a new instinct for using the modified organs. It 

 is enough to make us wonder if we really understand the nature of 

 biological adaptation. 



Though pupae that breathe free air ordinarily float at the surface 

 of the water, they can escape danger by darting around on the surface 

 or submerging quite as actively as the larvae by snapping movements 

 of the flexible and well-musculated abdomen. The large tail fins are 

 organs for increasing the motor efficiency of the abdomen. Func- 

 tionally they are comparable to the tail fan of a crayfish. The pupa 

 when swimming progressively on or below the surface kicks back- 

 ward with its abdomen and propels itself forward, but the crayfish 

 does just the opposite. When the pupa swims downward in the water, 

 however, it goes tail first, and thus maintains its floating position. 

 If it remains inactive it passively rises to the surface, otherwise it 

 swims up by abdominal movements. 



The head and mouth parts. — The head of the pupa (fig. 16 A, H) is 

 closely attached to the lower anterior angle of the thorax, with its 

 true dorsal surface directed anteriorly. It retains nothing of the 

 structure of the larval head. The long, many-jointed antennae curve 

 upward and backward beneath the lower edges of the wings. The 

 large, black compound eyes (fig. 15 D, E) are conspicuous beneath 

 the cuticle, and between them the clypeal region (E, Clp) makes a 

 prominent bulge on the face. Posteriorly the head is produced into 

 a long, tapering proboscis that lies beneath the thorax with its end 

 upcurved behind the lower legs (fig. 16 A, Prb). The component 

 elements of the proboscis are closely adherent (fig. 15 E), but are 

 easily separated (F). Along the lower side is the relatively thick 

 labrum (E, Lm) which is continuous from the clypeus. Flanking the 

 labrum are the very delicate slender mandibles (Md), and bordering 

 the mandibles are the maxillae (Mx). The wide base of each maxilla 

 bears a free, tapering palpus (F. Pip). On the posterior (upper) side 

 of the proboscis is the soft, slender, tubular labium ending in a bifid 

 tip (F. Lb). There is no free hypopharynx in the young pupa. 



As we have seen, the hypopharynx is not separated from the labium 

 in the larva, and the two parts go over still united into the pupa, with 

 the salivary duct enclosed between them. In most adult insects the 

 hypopharynx is an independent suboral lobe, and the salivary duct 



