2,(^ INSECTS. PHYLUM ARTHROPODA 



imagines). The insects may be divided into a number of groups 

 according to their Hfe-histories ; these groups correspond only 

 partially to the subclasses of the formal classification. 



rhe Ametabola are those which have no larva and no meta- 

 morphosis ; the form hatched from the egg is not mature, but 

 differs from the adult in hardly anything but the reproductive 

 system, which gradually develops. This group includes the 

 primitive wingless insects such as silverfish and springtails, and 

 a luimber of wingless members of other groups, such as the lice 

 and the workers of termites. 



In the Paurometabola the young instars differ from the adults 

 not only in having undeveloped gonads but in not possessing 

 wings ; they have, however, no positive characters of their own, 

 and are not strictly larvae although often called so. They are 

 known as nymphs, which, since the word means, in origin, 

 maidens fit for marriage, is a somewhat inappropriate term for 

 sexually immature animals. Development consists in the growth 

 of the reproductive organs, and in the gradual formation of 

 wings. In this group comes the cockroach, and most of the orders 

 of insects which in the formal classification are called Exoptery- 

 gota. There is no metamorphosis, although entomologists often 

 speak as if there were. 



The Hemimetabola are similar to the Paurometabola, but 

 differ in that the young stages live in water and have peculiar 

 features, suited to the habitat, which are absent from the adult ; 

 they are, therefore, true larvae. They are often called nymphs, 

 but a better term is naiads, which points out both their similarity 

 to and difference from the nymphs of the Paurometabola. (In 

 Greek mythology the naiades were water-nymphs.) There is 

 necessarily a metamorphosis, which takes place as the animal 

 emerges from the water. In the mayflies, for instance, the naiad 

 crawls up a plant stem into the air, and goes through an ecdysis 

 which produces a winged instar. After a short time a final moult 

 produces the imago. There is here foreshadowed the connection 

 of metamorphosis with moulting so notable in the higher insects. 

 The Hemimetabola include only the stoneflies, dragonflies, and 

 mayflies. The sole larval characters of the first are the gills, but 

 the naiads of the last two possess also mouth-parts different 

 from those of the adult. 



The Paurometabola and Hemimetabola are usually grouped 

 together as Heterometabola, an assemblage of orders almost 



