AND OTHER INJURIOUS INSECTS OF I907 AND I908. I79 



The Head in General: The head of aphids is small, either 

 transverse or quadrate in outline and is closely articulated with the 

 prothorax. The anterior margin between the bases of the antennae 

 called the frontal margin is of value in distinguishing genera and other 

 groups. Three different types of head may be recognized in the 

 Aphididcc. First, there is the primitive transverse form found in 

 Lachnus and allied genera ; where the frontal margin is straight and 

 there are no frontal tubercles. Then there is the cjuadrate form of the 

 higher genera where the frontal margin is concave. This is due to the 

 angles of the head on which the antennae are situated being drawn 

 out into frontal tubercles. These tubercles are of value in distinguish- 

 ing the higher genera of the Aphididcu, but do not have as great 

 taxonomic value as is usually attached to them, as they grade com- 

 pletely into those species of Aphis characterized by the absence of 

 frontal tubercles. Sometimes the inner margins of the frontal tuber- 

 cles are provided W'ith a protuberance or swelling; they are then said 

 to be gibbous. If this protuberance is conical or spine-like they are 

 toothed. The third or convex type is found in the gall-makers. Here 

 the frontal margin is convex and the anterior angles of the head instead 

 of being drawn out are rounded off. As a result the antennse and 

 anterior ocellus are situated a little more toward the ventral side than 

 in the other forms. In Lachnus and P. vagabiiudus the head is divided 

 by a middorsal suture. 



The Eyes: In the larva there is a group of three ocelli with 

 elliptic lenses on each side of the head, which undoubtedly represent 

 the larval eyes of other insects. They appear very early in the embryo 

 and may be seen as bright red spots through the abdominal wall of 

 the viviparous female. The larval eyes of Aphids are remarkable in 

 that they persist in the adult as ocular tubercles alongside the compound 

 eyes. The compound eyes begin to develop in the embryo just in front 

 of the larval eyes, but cannot be seen externally at that time. They 

 are precocious in their development and are present in the larva. They 

 appear externally and are probably functional after the first moult. 

 The lenses are hemispherical and are few in number, about forty at 

 this time. In the adult they range in number from seventy-five to two 

 hundred and fifty. The dorsal ocelli are three in number, one located 

 at the inner margin of each compound eye, and one located on the 

 frons, either on the frontal margin or ventral to it. They are not 

 present in the apterous forms. The compound eyes and the dorsal 

 ocelli, which are primarily correlated with the development of wings, 

 are not present, or are rudimentary in some of the subterranean forms, 

 where they have been lost from lack of use. 



