IV] THE LARVA OR NYMPH 75 



margin. As the wings develop, the synthorax increases in size 

 out of all proportion with the pro thorax. The latter, however, 

 is never so small in the larva as in the imago. In the later larval 

 stages, the mesopleurae grow forward to meet each other in front 

 of the wing-sheaths. The latter lie parallel to one another, pressed 

 flatly against the abdomen. Just before metamorphosis, the 

 swelling of the pleurae becomes so pronounced that the wing- 

 sheaths stand up vertically above the abdomen. 



6. Division of the tarsus. At the third instar a small basal 

 piece separates off from the tarsus. At the fourth or a later instar, 

 a third small joint is added in the same manner. In the Gomphinae, 

 the pro- and mesotarsi remain two- jointed. 



7. Growth of the appendix dorsalis. In those Anisoptera 

 (e.g. Anax) where this appendix is at first rudimentary, increase 

 of size takes place only slowly. At the eighth or ninth instar, the 

 appendix dorsalis may be almost as long as the cerci. It generally 

 becomes grooved ventrally, and is often bifid distally. In some 

 forms it never becomes long. 



8. Changes in the rectal gills of Anisoptera. These are fully 

 dealt with in chap. ix. All Anisopterid larvae start with a simplex 

 system of gills of the undulate type. In the Aeschninae this does 

 not reach the duplex form until the seventh instar. In the 

 Libellulidae, the lamellate duplex form is assumed at the third 

 instar. 



9. Changes in the caudal gills of Zygoptera. The slender pro- 

 cesses of the young larva become triquetral at the second or third 

 instar. By flattening (either laterally or dorso-ventrally) sub- 

 sequent instars form the lamellar gill. Saccoid gills are formed 

 by inflation of the original triquetral form. In Calopteryginae, 

 a differentiation in size and shape takes place in the median and 

 the lateral gills. 



10. Changes in the nervous system. The ventral ganglia do 

 not increase much in size as the larva grows. Thus they become 

 smaller and smaller relative to the segments containing them. 

 The cords connecting consecutive pairs of ganglia rapidly increase 

 in length. A comparison of figs. 54 and 59 shews the extent of 

 this change. The ganglia tend to move forward in each segment. 

 Finally, the ganglion of the second segment moves up into the 



