20 ANA TOMV AND MORPHOLOGY OF INSECTS. 



tively rudimentary condition of those which are needful for the 

 protection and dispersion of the individual.* 



The Resting^ Larva. — Before each ecdysis the larva ceases to 

 feed, generally seeks a place of safety, and becomes languid and 

 motionless. The duration of the period which elapses before it 

 again commences active life is greater the more profound the 

 changes which occur at the moult. This phenomenon is espe- 

 cially marked before the formation of the nymph in the 

 Metabola ; some caterpillars remain inactive for months before 

 they moult and reveal the contained nymph, which is deve- 

 loped during the resting period. In the Diptera and Hymen- 

 optera the larva skin frequently becomes modified and replaces 

 the silk cocoon which many larvas spin for the protection of 

 the nymph. 



The Pupa and Nymph. — The general use of these terms is mis- 

 leading. A pupa has been defined as a resting nymph, a nymph 

 as an active nymph. If they were used in agreement with this 

 definition no further objection could be made than that the 

 term pupa is unnecessary, but the resting nymph of a lepi- 

 dopterous insect is not the same thing as the pupa of a fly. 

 The pupa of the fly is a resting larva, and the nymph, which is 

 similar to the resting nymph of a butterfly, is developed within 

 it. I shall, therefore, use the term ' pupa ' for the final stage 

 of the resting larva {pupa coardata), and shall always use the 

 term ' nymph ' for the so-called pupa obtecta, which is the im- 

 perfect imago. 



In insects with an incomplete metamorphosis, the manner in 

 which the wings are developed is very instructive, and forms 

 a key to the manner in which the nymph is formed in the 

 higher, or more specialised, forms of insects. 



Previously to the ecdysis, when the rudimentary wings be- 

 come external organs, the hypodermis separates from the 

 cuticular layer of the integument in the region where the rudi- 

 mentary wing appears, and the wing is formed as an out- 

 growth from the surface of the separated hypoderm beneath 



* See also Brauer, F., ' Ueber die Verwandlung der Insecten.' Verb. 

 Zool.-Iiot. Gesellsch., Wien, Bd. xi,x. 



