16 ON SOME INSECT DEFORMITIES. 



insects retaining the larval head may depend on the larger mortality of 

 their caterpillars. 



There is very little known concerning the physiological and mechani- 

 cal processes shortly before or during the act of transformation of 

 arthropods. Nearly all entomological works state that the larvae moult 

 or change their skin several times, that the larva 1 become restless some 

 days before the change, stop eating, and desert their food ; later, the 

 skin splits, and the insect perfects its transformation. 



I believe there exist few men who have not seen and observed once 

 in their life this wonderful spectacle. The proceedings are so common, 

 and always so easily performed, that observers are not induced to think 

 about the manner in which transformation is effected, nor about the 

 mechanical acts providing the possibility of such a change. Concerning 

 the mechanical acts, so far as I know, nothing is published. An animal, 

 or even a man. covered with an artificial skin well fitted to the whole 

 body, obliged to go out of the skin through an aperture made of similar 

 size and relation as that in insects, would scarcely be able to do it with- 

 out a violent use of the limbs. Insects use their limbs very little or 

 not at all in the beginning of transformation, but nature has provided 

 some help in the necessary coincidence of certain physiological pro- 

 ceedings just at the time of transformation. 



I have observed many times and in different insects, before transfor- 

 mation, a very accelerated and excited action of the dorsal vessel. The 

 same fact is recorded by other observers, for instance by Mr. Weismann 

 and Mr. W. Blasius. After observations of Mr. W. Blasius (Zeitschr. 

 f. wiss. Zobl., Vol. XVI, pp. 135 - 177 I. during the transformation of the 

 caterpillar into the chrysalis, the action of the dorsal vessel increases 

 successively in the first three hours, and reaches its maximum in the 

 last half of the fourth ; after that time the action begins to decrease, 

 and becomes in the eighth hour equal to the action in the third hour. 

 The consequence of an accelerated action of the dorsal vessel is an in- 

 creased circulation of the blood, going from the tail to the head. This 

 sudden rush of an unusual quantity of blood to the head and the tho- 

 rax, without any corresponding arrangement for convenient emanation, 

 swells those parts, pushing them forward at the same time. Finally 

 the skin bursts, and one of the most important acts of the moult is per- 

 formed . 



I suppose that the frontal bladder, which is observed in transforming 

 Diptera and Odonata, is the consequence of the rush of blood ; never- 

 theless, an observation recorded by Weismann seems to disagree with 

 such a supposition. It should not be overlooked that some other purely 

 mechanical proceedings seem to accompany and help the propulsion of 

 the insect by the rush of the blood in a very easy manner. I have 

 described long ago a related fact in regard to the moult of Ephemera, 



