TRANSFORMATIONS OF THE INSECT. 65 



In the metamorphosis of Corethra., a Mosquito-like Fly, which 

 is active both in the larva and pupa states, "the segments of 

 the larva are converted directly into the corresponding seg- 

 ments of the body of the imago, the appendages of the head 

 into the corresponding ones of the head of the imago ; those 

 of the thorax are produced after the last moult of the larva 

 as diverticula of the hypodermis round a nerve or trachea, 

 from the cellular envelope of which the formation of tissue in 

 the interior of the appendages issues. The larval muscles of 

 the abdominal segments are transferred unchanged into the 

 imago ; the thoracic muscles peculiar to the imago, as also 

 some additional abdominal muscles, are developed in the last 

 larval periods from indifferent cellular cords which are indi- 

 cated even in the egg. The genital glands date back to the 

 embryo, and are gradually developed ; all the other systems of 

 organs pass with little or no alteration into the imago. Fatty 

 body none or inconsiderable. Pupa-state short and active." 

 (Weismann.) 



As the two types are most clearly discriminated by the 

 presence or absence of true imaginal disks, Weismann suggests 

 that those insects which undergo a marked metamorphosils 

 niight be divided into Insecta discoid (or Insects with imaginal 

 disks) , and those without, into Insecta adiscota. 



The metamorphosis of Corethra may prove to be a type of 

 that of all insects which are active in their preparatory stages ; 

 and that of Musca typical of all those that are quiescent in the 

 pupa-state, at least the Lepidoptera and those Diptera which 

 have a coarctate * pupa, together with the Coleoptera and those 

 Neuroptera in which the metamorphosis is complete, as Phry- 

 ganea, Hemerobius, etc. 



The transformations of the Humble-bee are easily observed 

 by taking a nest after the first brood have matured, when w^e 

 shall find individuals in all stages of development from the 

 larva to the imago state. The figures below show four stages, 

 but in reality there is every gradation between these stages. 



* The larvae of some of the higher Diptera spin a slight cocoon, while the true 

 flies, such as the Muscids and Syrphidse, etc., change to pupae within the larva 

 skin which contracts into a cylindrical "puparium" corresponding in use to the 

 cocoon; such pupae are called "coarctate." 



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