288 INVERTEBEATA CHAP. 



the embryo of Blatta, with its well-developed abdominal appendages, 

 abundantly proves. The so-called nymph, which emerges from the 

 egg without wings, or with mere rudiments of them, corresponds to 

 the pupal stage of higher insects, the difference being that the nymph 

 is active and seeks its own food. 



The reason why the caterpillar stage has become embryonic in 

 the lower insects is probably closely akin to the reason why the 

 development of the egg in an extremely primitive form like Peripatus 

 takes place entirely within the maternal body. It may perhaps be 

 expressed by a single phrase, " the pressure of modern competition." 

 The caterpillar-like form of larva would not stand the remotest chance 

 of survival unless the mother sought out for it a suitable environ- 

 ment, with plenty of food near at hand and easy to get. Where the 

 mother has not sense enough to do this, either the stock must die 

 out or the corresponding stage must be passed over within the egg 

 membrane. 



The active nymph which emerges from the egg is hard enough 

 put to it to survive. In the case of Odonata and Ephemeroptera, to 

 take instances, it is driven to seek shelter in an aquatic life, and may 

 be driven to develop organs like tracheal gills, which bring oxygen 

 to the tissues by extracting it from the water and passing it by 

 diffusion into air-tubes deprived of external openings, and from these 

 to the blood a'roundabout plan which betrays the secondary nature 

 of the aquatic life of these nymphs. 



In general it may be said, that since the invention of birds and 

 bats the life of a winged insect has become a much more perilous one 

 than formerly, and to this circumstance we attribute the fact that 

 there is a strong tendency to shorten the life-period of the imago and 

 prolong that of the larva. Indeed it often occurs that the imago takes 

 no food, or, as in all Lepidoptera, only food which is incapable of 

 prolonging life, since it is but saccharine. In the same way and for 

 the same reason, when sex union and egg-laying are accomplished, the 

 imago dies, no matter how securely it may be screened from enemies. 



The larval stage of Insecta, therefore, agrees in its fundamental 

 nature with the larval stages of other animals, and, we may be assured, 

 has not been secondarily intercalated in the life-cycle : it represents a 

 Thysanuran ancestor, but in the majority of insects it has undergone 

 great secondary modification, and in the so-called Hemimetabola, 

 like Orthoptera, it has become embryonic, and is passed through 

 while within the egg membrane. 



LITERATURE REFERRED TO 



P ROTOTR ACH E ATA 



Sedgwick, A. The Development of the Cape Species of Peripatus. Quart. Jotiru. 

 Mic. Soc., vols. 25-28, 1885-1888. 



Sheldon, L. On the Development of Peripatus novae-zelandiae. Quart. Journ. Mic. 

 Soc., vols. 28-29, 1888-1889. 



