EYES OF LARVA 87 



ridge the parts supporting the mental and labial structures turn abruptly to the 

 hypopharynx and pharyngeal opening. Medianly, and immediately behind the 

 ridge, are four successive chitinous structures. In a ventral view these are in 

 order: (1) a delicate roughly triangular plate with a marginal hair-fringe and 

 produced into a long tooth at the middle; behind this is (2) the "mental 

 sclerite," a heavily chitinized, dark, roughly triangular, dentate plate ; and back 

 of this (3) a complicated plate with teeth and spine-like structures, the labium; 

 farther back, at the margin of pharynx is the hypopharynx, a simple chitinous 

 cone. 



The first two plates are excrescences of the mentum ; the labium is attached 

 to the mentum on the inner side by a flexible membrane. The labium, although 

 complex in structure, allows none of the typical parts of that organ to be recog- 

 nized ; it presents very different aspects in dorsal and ventral view. It can be 

 drawn back by two muscles attached to its outer angles. 



The mental plate is conspicuous in preparations of mosquito larvgs by its dark 

 color and serrate anterior margin and is frequently alluded to in systematic 

 writings as the " labial plate." Miall and Hammond have termed this plate the 

 submentum. 



THE EYES. 



The organs of sight of the mosquito larva consist of two pairs of eyes. In most 

 insects vdth complete metamorphosis the eyes of the larva (when present) are 

 larval organs, independent of the eyes of the imago ; these latter only develop 

 towards the close of the larval period and do not become functional until the 

 imago-state is reached. In the mosquito, however, and in other Nemocera, the 

 eyes of the larva are compound in character and are those of the future imago ; 

 they are retained throughout the different stages. Ocelli, like those of other 

 insect larvae, are absent. The two pairs of eyes are situated laterally, a small 

 pair posteriorly and just in front of these are the large eyes which become the 

 compound eyes of the imago. The large eyes in the advanced larva are crescent- 

 shaped, the concave side turned posteriorly and the horns of the crescent ex- 

 tending backward over the dorsal and ventral surfaces of the head. 



When the larva first hatches only the smaller posterior pair of eyes are present 

 and before hatching these are already visible through the egg-shell as two reddish 

 spots. The character of the eyes at the time of hatching, and their subsequent 

 development, are described by Zavrel as follows : 



" The rose-red to brown pigment is differentiated into three portions : two 

 round ones and one in the shape of a band. With high magnification one finds 

 structures upon its margin similar to the lenses of a compound eye, which, how- 

 ever, are here only faintly indicated. Its nerve forms a small round ganglion in 

 front of the brain. On the fourth day the band or sickle-shaped main eye 

 appears in front of the accessory eye ; first it consists of rust-brown ommatidia, 

 later it is pigmented black. The entire main eye does not develop at once. 



" In young pupse the main eye is in the form of a band pointed at the ends. 

 It is always pigmented deep black and one also finds scattered groups of black 

 pigment in its vicinity. One can never detect that it is composed of separated 

 red ommatidia, as it is in the young pupa of Anopheles. Only later does it take 



