74 THE LARVA OR NYMPH [CH. 



approach very closely to one another. They remain surprisingly 

 small in Libellulidae, but become raised up on pyramids in the 

 higher forms, so as to stand clear of the huge mask (fig. 29 P). 



2. Development of the ocelli. These are absent during most of 

 larval life. During the last few instars, their rudiments appear; 

 and, in the Zygoptera at least, they are well-formed in the full- 

 grown larva. They do not appear to become functional until 

 metamorphosis. 



3. Increase in the number of joints of the antennae. The 

 antennae at first are three-jointed, there being a short broad base 

 or scape, a short but distinct pedicel, and a single long distale. At 

 the second ecdysis, the distale divides into two. At the next, the 

 more basal of these two again divides. At the fifth or sixth 

 ecdysis, the more basal of these again divides. There are thus, 

 by then, six joints, four being distalia. Either at the last or 

 penultimate ecdysis, the apical distale divides into two. Thus 

 the full-grown larva usually has 7- jointed antennae, as in the 

 imago. In Gomphinae, no sub-division takes place after four 

 joints are formed. Either the fourth or the third joint becomes 

 more or less swollen; in the latter case the fourth may become 

 only a vestigial knob. 



4. Changes in the form of the labial mask. These differ much 

 in the various groups [19]. The general tendencies are (a) more 

 complete fusion of parts, (6) repression of galea and lacinia, 

 (c) growth and strengthening of the movable hook, (d) increase 

 in the number of setae, in setigerous forms. In Calopteryx the 

 original cleft in the median lobe becomes intensified instead of 

 closing up. In some species of Anax, setae are at first present 

 and afterwards lost. 



5. Changes in the shape of the thorax. These are the most 

 important of all the changes that take place in larval life, since 

 they are correlated with the development of the wings. The 

 pleural ridges begin to thicken very early (4th to 5th instars). 

 At the next instar, the wing-rudiments appear as tiny buds 

 growing backwards from these ridges. At each ecdysis, these 

 buds increase in size, and soon become triangular flaps. The 

 pleural ridges take up an oblique position. Thus the hind wing- 

 sheath gradually overlaps the fore, until it covers all but the costal 



