DEVELOPMENT OF THE IMAGO. 367 



The absence of wings in all insect larvae points back to the 

 primitive nature of the Thy§anura, and many insect larvae actually 

 agree closely in appearance with, the members of this genus. The 

 Campodeiform larvae, the importance of which was specially pointed 

 out by Brauer (No. 145), would thus represent the larval type 

 which has most nearly retained the primitive character. As the 

 chief characteristics of this type Ave would name : biting mouth- 

 parts, jointed antennae, thoracic segments more or less resembling 

 the abdominal segments, well developed thoracic limbs, a long, 

 slender, ventrally compressed form of body, and two jointed pro- 

 cesses (cerci) at the end of the abdomen. This type is fairly 

 accurately adhered to by the larvae of the Ephemeridae, Perlidae, 

 many Xeuroptera, and many Coleoptera. 



The metamorphosis of the Insecta, as a rule, is more sharply 

 marked in the higher orders, the separate stages being more unlike 

 one another, and the transition between them not being gradual. 

 "We must therefore regard incomplete metamorphosis as the more 

 primitive condition^ and complete metamorphosis as a higher grade 

 of individual development acquired in the Insecta. Consequently, 

 the larvae of the Metabola must all be considered as derived forms. 

 But in the Hemimetabola also, certain characters, phyletically con- 

 sidered, must also be regarded as new acquisitions, e.g., the presence 

 of a so-called closed tracheal system and of tracheal gills in many 

 aquatic larvae, since, in all probability, this manner of life must 

 be considered as only recently adopted. 



"While, therefore, little importance attaches, phyletically, to the 

 larval forms of Insects, certain features are perhaps of some value, 

 in so far as the acquired larval forms also show a tendency to repro- 

 duce the morphological characters of the ancestral forms. Among 

 the features which have thus come to the surface again, we have : 

 (1) the softer nature of the integument of the body-surface ; (2) the 

 less marked separation of the thorax from the abdomen ; (3) the 

 more uniform segmentation of the extremities ; (4) the absence of 

 the facet-eyes ; (5) the frequent occurrence of abdominal limbs. 



2. Development of the Imago. 



We have already (p. 355) pointed out the characteristic distinction 

 existing between the homomorphous orders of Insects on the one 

 hand, and the holometabolic forms on the other with regard to the 

 manner of development of the sexually mature (imaginal) condition. 

 In the first group, the development of the adult is accomplished 



