438 AN INTRODUCTION TO ENTOMOLOGY 



thus the body is lifted away from the leaf and is perched upon an 

 exquisite palisade of white wax (Fig. 509). 



The members of this family feed exclusively on the leaves of the 

 host -plants. With few exceptions they are not of economic impor- 

 tance ; and also with few exceptions, the injurious species are not wide- 

 ly distributed over the world as are many aphids and coccids. This is 

 probably due to the fact that as they live exclusively on leaves they 

 are not so liable to be transported on cuttings and nursery stock. 

 They are most abundant in tropical and semi-tropical regions. 



The adults present the following characters: The compound eyes 

 are usually constricted in the middle and in some species each eye 

 is completely divided. In some cases the facets of the two parts of a 

 divided eye are different in size ; it is probable that in such cases one 

 part is a day-eye and the other part a night-eye (see page 1 44) . The 

 ocelli are two in number; each ocellus is situated near the anterior 

 margin of a compound eye. The antennse are usually seven-jointed. 

 The labium is composed of three segments. The fore wings are larger 

 than the hind wings; when at rest the wings are carried nearly 

 horizontally. The venation of the wings is greatly reduced; the 

 maximum number of wing-veins found in the family is in the fore 

 wings of the genus Udamoselis (Fig. 510). The three pairs of legs 

 are similar in form; the tarsi are two-jointed; and each tarsus is 

 furnished with a pair of claws and an empodium or paronychium. 

 The anus opens on the dorsal wall of the abdomen at some distance 

 from the caudal end of the body and within a tubular structure, 

 which is termed the vasiform orifice. A tongue-like organ, the lingula, 

 projects from the vasiform orifice; and at the base of the lingula 

 there is a broad plate, the operculum; the anus opens beneath these 

 two organs. 



In this family the type of metamorphosis corresponds quite 

 closely with that known as complete metamorphosis; consequently 

 the term larva is applied to the immature instars except the last, 

 which is designated the pupa. 



The eggs are elongate-oval in shape and are stalked. The larvae 

 during the first stadium are active, after which they remain quiescent. 

 There are four larval and one pupal instars. The wings arise as 

 histoblasts in the late embryo, and the growth of the wing-buds 

 during the larval stadia takes place inside the body-wall. The 

 change to the pupal instar, in which the wing-buds are external, 

 takes place beneath the last larval skin, which is known as the pupa- 

 case or puparium. In many descriptions of these insects only three 

 larval instars are recognized, the fourth being described as the pupa. 

 As the change to a pupa takes place beneath the last larval skin, the 

 puparitmi, and as the adult emerges through a T-shaped opening in 

 the dorsal wall of the puparium, the pupa itself is rarely observed. 



Parthenogenesis occurs in this family; but according to the 

 observations of Morrill, unfertilized eggs produce only males. 



As with the adults, the anus of the immature forms opens in a 

 vasiform orifice on the dorsal aspect of the body at some distance 



