METAMORPHOSIS. 355 



II. Metamorphosis. 

 1. The Larval Forms.* 



The Thysanura and the Collembola emerge from the egg in a form 

 resembling that of the adult, so that there can here be no question 

 as to the occurrence of a metamorphosis ; they may consequently be 

 described as true Ametabola (Insects -without metamorphosis). 



All other Insects, on the contrary, pass through a true metamor- 

 phosis. When they leave the egg they are distinguished from the 

 adult not only by their smaller size, but also by the absence of the 

 wings. Many Insect larvae differ further from the adult {imago) in 

 a number of ways. 



If we compare the young forms (larvae) of Insects when hatched 

 with- those of many Crustacea which leave the egg as Nauplii, we 

 firtd a marked distinction between them. In the Insects, the typical 

 number of segments is developed in the embryo; the limbs also, 

 and the rudiments of the most important organs are already present. 

 Only the wings are wanting. In other respects, the young emerging 

 from the egg has the characteristics of a well-formed Insect. There 

 is no doubt that the Thysanura, and among these the Campodea 

 especially (Fig. 193), stand very near the fundamental form of 

 tin's Avingless larva. We have in the Thysanura undoubtedly the 

 most primitive living representatives of the class of the Insecta. 

 But we must'not lose sight of the fact that, even amongst these 

 forms, many systems of organs (e.g., the tracheal system) have 

 undergone reduction, possibly on account of the small size of the 

 body. 



The orders of the Insecta may be grouped in two divisions, 

 according to the manner of their metamorphosis. To one of these 

 groups belong those orders which we are accustomed to regard as 

 the more primitive on account of their organisation ; among these 

 we find some with an invaginated germ -band, thus suggesting a 

 connection with the Myriopoda. The larva here passes gradually, 

 by a series of^stages each^marked by an ecdysis, into the imaginal 

 form. During these stages the rudiments of the wings grow out, 

 increasing in size gradually. Metamorphosis thus here takes the 

 form of onward growth within the limits of the segmentation and 

 rudimentary organisation already present. Such development is 

 distinguished as incomplete metamorphosis, and the Insects belonging 

 to this type are known as the Homomorpha. 



* In the following account we have chietly followed Lvbbock (No. 156) and 

 Bkaueii (No. 146). 



