26 MOSQUITOES OF NOETH AMERICA 



be any modification of the chitin overlying them, and of course there is no trace 

 of the " rod and cone " structure of the compound eyes. \Ye shall revert to 

 these vestigial eyes again in our discussion of the eyes of the larva. 



THE ANTENN/E. 



The antennas are composed of fifteen segments. The basal segment is greatly 

 reduced and not distinctly visible, and therefore in descriptive work the antenna^ 

 are considered as fourteen-jointed. 



Child, who demonstrated the presence of the basal rudimentary segment, 

 states that the antenna is made up of sixteen elements, but he was in error with 

 the number of joints in the shaft. In all Culicidae examined by us the number 

 of antennal joints, both in the male and female, have been found the same, 

 fifteen, if one counts the basal reduced joint. The second joint dift'ers from all 

 the others ; it is large, globose, with a cup-shaped hollow on top, and upon the 

 floor of this hollow the third joint is inserted. In descriptive work this joint is 

 usually termed the torus. This joint constitutes a highly specialized sense- 

 organ, generally accepted to be an organ for the perception of sound waves. 

 Farther on we will give the details of structure of this organ and a discussion 

 of its functions. The joint differs considerably in size in different species and 

 in the two sexes. The torus is largest in the male and this sexual difference is 

 most marked in those species in which the male antennas are strongly plumose. 

 The tori are usually naked and smooth ; often they are pruinose and sometimes 

 more or less densely clothed with scales; there may be small scattered hairs 

 present upon the surface. 



The outer thirteen joints constitute the shaft and are slender, more or less 

 elongate, and cylindrical ; they are more or less pubescent and each bears a whorl 

 of hairs. The appearance of the shaft, and the details of structure of the com- 

 ponent joints, differ considerably with the species and groups of species and 

 particularly in the two sexes. 



The simplest form, perhaps, is that which obtains in the females of Culex, 

 Aedes, Anopheles, and related forms. In these the joints are elongate, cylindri- 

 cal, subequal, with a whorl of long hairs at the base ; the fourth joint (the second 

 of the shaft) is the shortest, each succeeding joint progressively longer. The 

 first and the last joints of the shaft differ most from the others. The first joint 

 of the shaft is longer than the succeeding ones and usually more or less swollen ; 

 it also differs from the others in lacking the basal whorl of hairs and bears in- 

 stead some irregularly inserted hairs near its middle. The terminal joint is the 

 longest of the series and has a pointed apex; besides the basal whorl it has a 

 whorl of sparse hairs just below the tip. 



In some forms there is an apical whorl of smaller hairs upon all the joints of 

 the shaft, but these do not show the regularity of insertion of the basal whorl 

 of hairs. Besides these coarse hairs of the whorls there are many fine hairs, in- 

 serted in sensory pits, scattered irregularly over the surface of the joints ; they 

 vary in coarseness and abundance with the species. In addition scales may be 

 present on some of the joints — particularly the first joint of the shaft is some- 



