HETEROMORPHA. 359 



wings are present, passes into a stage closely resembling the imago (sub-imago) ; 

 this is distinguished from the preceding stages by the fact that, during it, no 

 nourishment is taken. In this moult, the stigmata and stigmatic branches are 

 definitely opened, and the tracheal gills, becoming constricted at their points 

 of insertion, are cast off and remain in the empty skin (exuviae) of the last 

 nymph-stage. One more moult on dry land leads from the sub-imago-stage 

 to the form of the imago. 



The larvae of the Odonata are in some cases elongate and very like the imago, 

 in others they are distinguished from it by their more compact form. AH 

 Odonatan larvae are characterised by the remarkable modification of the lower 

 lip which forms the protrusible " mask " (seizing pincer). 



The respiratory organs are variously modified in the different genera. The 

 closable stigmata on the thorax and abdomen of the larva (Hagen) appear to be 

 used chiefly for giving off air, but the older Libellulid larvae also breathe in air 

 through the thoracic stigmata (Dewitz). The tracheal gills are internal in 

 Aeschna and Libellula, being situated as folds or outgrowths on the walls of the 

 rectum {intestinal gills) ; in the Agrionidac, three branchial leaves are found on 

 the last abdominal segment. These, as well as the intestinal gills of the 

 Libellulirfae (Hagen), are cast off when the larva passes over into the imaginal 

 form. In Euphaea, which is distinguished by the presence of abdominal 

 appendages, long, conical gills are found on either side of the body near the 

 stigmata on the second abdominal segment to the eighth (Hagen). 



Plecoptera. In so far as, in the Perlidae, the tracheal tufts (Fig. 178 A, k) 

 are retained in the imago, and the actual metamorphosis consists only of the 

 gradual growing out of the wings, this family, strictly speaking, should be 

 classed among the Paurometabola. The larvae are Campodeiform (Fig. 178 B) 

 and their respiratory organs appear in various positions, as lateral gill-tufts 

 (Fig. 17S A, k) at the sides of the thorax, as pro-sternal gills on the first ventral 

 shield, at the sides of the anal aperture, or on the lateral margins of the 

 abdomen. We are justified in classing the Perlidae among the Hemimetabola, 

 by the fact that the branchial tracheal gills in the imagines do not function as 

 such, but are retained in a shrivelled and vestigial condition. 



B. Heteromorpha. 



The larvae of the Insects that belong to this group are very 

 different in appearance from the imagines. Some of them recall 

 the Campodeiform larva, but they are often modified in adapta- 

 tion to a definite manner of life, and frequently degenerate through 

 the preponderance of the vegetative functions and the partial 

 suppression of free locomotion in consequence of more or less 

 parasitic life. The climax of degeneration is reached in the limb- 

 less and eyeless "maggot" with reduced masticatory organs. In 

 most cases the larva differs entirely from the adult in its manner 

 of life. "We must regard complete metamorphosis as a more 

 specialised form of the process of development, an acquired differ- 

 entiation in larval life distinguishing the highly-developed, but 

 probably, according to their origin, younger orders of Insects as 

 contrasted with, the Homomorpha. 



