246 Arthropoda. 



regular occurrence of parthenogenesis is found in many social 

 Hymenoptera wliere the males originate from unfertilised, the females 

 from fertilised, ova. Not infrequently, parthenogenetic reproduction 

 alternates regularly with the usual sexual mode, so that there is 

 heterogony ; either each unisexual generation alternates with 

 a true sexual one (Gall-flies), or the sexual is followed by seyeral 

 parthenogenetic generations {Aphides). Usually, the parthenogenetic 

 generations differ somewhat from the other; sometimes, if there 

 are several such successive generations, these also differ from each 

 other. 



In some of the Diptera (Cecidomyia) eggs may arise prematui'ely in the 

 larva, and develop direct without fertilisation into viviparous lai-vse, the parent 

 lai"va dies, whilst the yoimg ones grow, and either give rise in the same way to 

 another generation or become perfect insects. Thus parthenogenesis may oecm- 

 precociously, when there is otherwise no sexual reproduction. This process is 

 known as psedogenesis. 



Insects are usually oviparous, but the parent is most solicitous in 

 ensuring that the newly-hatched larvae shall be well supplied with 

 nourishment ; this is generally accomplished by laying the eggs in 

 places where suitable food is present, but occasionally by the collec- 

 tion of a supply of food where the eggs are deposited (certain Dung- 

 beetles, Sand-wasps) ; more rarely the parental instinct is more 

 highly evolved, and the female brings a fresh supply of food each day 

 to the brood. In a few Insects the eggs are only laid when the 

 embryo is about to be hatched ; others are viviparous, embryonic 

 development being completed within the oviduct. A peculiar 

 arrangement is met with in the Forest-flies {Hippobosca) , in which not 

 only is the egg completely developed within the oviduct, but the larva 

 remains there for some time feeding upon the secretion of certain 

 glandular appendages. 



The majority of Insects on leaving the egg do not resemble the 

 adult, but undergo a metamorphosis; only in a few cases, e.g., 

 Lice and various other apterous forms, are the changes so insignificant 

 that it is impossible to speak of a true metamorphosis. Meta- 

 morphosis may be more or less thorough ; two chief types may, there- 

 fore, be distinguished, complete or holometabolous, and in- 

 complete or hemimetabolous. 



In hemimetabolous Insects (Orthoptera, Ehynchota) the newly- 

 hatched larva differs chiefly from the adult or imago only in that it 

 is apterous. In other respects the differences are slight, the 

 number of joints in the antennae may be fewer, the head relatively 

 larger than in the adult and so on. The transition from the first larval 

 stage to the adult occurs gradually ; wings begin to appear,* small at 

 first, but increasing at each ecdysis, until after the last moult they are 

 fully formed and functional : at the same time the other portions of 



* Such larvse, possessed of incipient wings, are often termed nymphs. 



