LARVAE AND METAMORPHOSES 31 



skeleton and jointed limbs, and in their development show a series 

 of moults. No life -history in the animal kingdom is more surprising 

 than that of a fly like the blow-fly. The eggs are laid on animal 

 matter, and the flies, no doubt attracted by the smell, prefer matter 

 that is just beginning to soften with putrefaction. The eggs hatch 

 out into the little brown -headed white maggots known as gentles 

 (Fig. 15). They have a pair of strong jaws with which they devour 

 the animal matter in which they are living, a segmented body clad 

 in a tough leathery skin, and no trace of limbs. They moult two 

 or three times without changing their shape, but growing larger, 

 and soon after the last moult, contract into a quiescent oval body, 

 covered with the skin of the larva which has become dry and brown. 

 After some days passed in this f^^^-sp^^ 

 motionless state, the brown skin ; , ; >; ?^-^ 



splits, and the fully formed adult | .>:,.". 

 fly emerges, and in a few minutes V > - - 



is winging its way through the air, __ .^ ,^^^ 



as unlike the worm-shaped larva : :^- , ^ 



as any creature could be. With ) 



the exception of the nervous ^M^ -K^ 



system and parts of some other _ 



J . . , , n , FIG. 15. Larva (upper figure) and 



organs, it seems as if the whole of pup a (lower figure) of Blow-fly, 

 the organs inside the hardened skin enlarged)* ""* PACKARD; 

 of the larva melted down and 



became rearranged to form the very different organs of the adult. 

 Patient and extremely difficult dissections, however, have shown 

 that there is an intelligible order in this transformation. Some 

 time before the fly emerges it is surrounded by two delicate and 

 transparent skins. The inner of these, if we could imagine it taken 

 out whole, plumped up with air, and dried, would have the appear- 

 ance of a fly with a head bearing antennae, eyes and mouth-organs, 

 a body with small wings and six-jointed legs, and a pointed abdomen, 

 but with all these organs and parts, and especially the wings, not 

 quite like those of a modern fly, but rather simpler. This skin is the 

 pale ghost of a former metamorphosis, of a true moult once passed 

 through by the ancestors of the flies, but now on its way to be 

 suppressed. The outer thin skin is the similar remains of a still 

 earlier moult, and its structure, although still fly -like, is less fly -like 

 than the inner skin. 



The development of a moth such as the well-known privet hawk- 

 moth carries the story a little further. The eggs are laid on the 



