64 THE CLASS OF INSECTS. 



insect be examined at intervals, a series of slight changes of 

 form may be seen, from the larva to the imago state. In some 

 cases each change is accompanied by a moult, as in the ' ' ac- 

 tive " Ephemera, where Lubbock counted twenty one moults. 



As a general rule, then, it may be stated that the body of 

 the larva is transformed into that of the imago ; ring answer- 

 ing to ring, and limb to limb in both, the head of the one 

 is homologous with that of the other, and the appendages of 

 the larva are homologous with the appendages of the imago. 



Weismann has shown that in the larva of the Meat-fly, Musca 

 vomitoria, the thorax and head of the imago are developed 

 from what he calls "imaginal disks." These disks are minute 

 isolated portions of the hypodermis, which are formed in the 

 embryo, before it leaves the egg, and are held in place within 

 the body-cavity of the larva by being attached either to nerves 

 or tracheae, or both. After the outer layer of the larva skin 

 dries and hardens, and forms the cask-shaped ptijjarhirn, the 

 use of which corresponds to the cocoon of moths, etc., these 

 imaginal disks increase in size so as to form the tegument of 

 the thorax and head. The abdomen of the Meat-fl}' , however, 

 is formed by the direct conversion of the eight hinder segments 

 of the body of the larva, into the corresponding segments of 

 the imago. 



Accompanying this change in the integument there is a 

 destruction of all the larval system of organs ; this is either 

 total or effected by the gradual destruction of tissues. Now 

 we see the use of the " fatty bod}^ ; " this breaks up, setting 

 free granular globules of fat, which, as we have seen in the 

 embryo, produces by the multiplication of cells the new tissues 

 of the pupa. Thus the larva-skin is cast aside, and also the 

 softer organs within, but the formation of new tissues keeps 

 even pace with the destruction of the old, and the insect pre- 

 serves its identity throughout. The genital glands, however, 

 are indicated even in the embryo, and are gradually developed 

 throughout the growth of the insect, so that this histolysis, or 

 destruction of tissues, is not wholly complete. The quiescent 

 pupa-state of Musca is long-continued, and its vitality is latent, 

 the acts of respiration and circulation being almost suspended. 

 (Weismann.) 



