INFLUENCE OF HEAT ON ANIMALS. 87 



visions made by Nature, in the physical or mental constitution of ani- 

 mals, for affording that influence. The development of heat around the 

 oviduct of the Serpent is a process over which the individual has no 

 control, being entirely dependent upon certain organic changes ; whilst 

 the imparting of warmth to its eggs by the Bird, either from its own 

 body or through artificial means, is committed to the guidance of its 

 Instinct, which same instinct leads it to suspend the process when it 

 is not necessary. 



126. Phenomena of an equally interesting and instructive character 

 may be observed in the history of the Pupa-state of Insects ; which, in 

 those that undergo a complete metamorphosis, may be almost charac- 

 terized as a re-entrance into the egg. In fact we shall obtain the most 

 correct idea of the nature of that metamorphosis, by considering the 

 Larva as an embryo, which conies forth from the egg in a very early and 

 undeveloped condition, for the sake of obtaining materials for its continued 

 development, which the egg does not supply in sufficient amount. When 

 these have been digested and stored-up in the body, the animal becomes 

 completely inactive, so far as regards its external manifestations of life ; 

 and it forms some kind of envelope for its protection, which may not be 

 unaptly compared to the shell or horny covering of the egg. Within 

 this are gradually developed the wings, legs, and other parts which are 

 peculiar to the perfect Insect ; whilst even those organs, which it pos- 

 sesses in common with the Larva, are for the most part completely 

 altered in character. When this process of development is completed, 

 the Insect emerges from its Pupa-case, just as the Bird comes forth from 

 the egg; then only does its Insect life begin, its previous condition 

 having been that of a Worm ; and the alteration of its character is just 

 as evident in its instinctive propensities, as it is in its locomotive and 

 sensorial powers. 



127. Now this process of development is remarkably influenced by 

 external temperature ; being accelerated by genial warmth, and retarded 

 by cold. There are many Larvae, which naturally pass into the Pupa 

 state during the autumn, remain in it during the entire winter, and 

 emerge as perfect Insects with the return of spring. It was found by 

 Reaumur, that Pupae, which would not naturally have been disclosed 

 until May, might be caused to undergo their metamorphosis during the 

 depth of winter, by the influence of artificial heat ; whilst, on the other 

 hand, their change might be delayed a whole year beyond its usual 

 time, by the prolonged influence of a cold atmosphere. In order to 

 hasten the development of the pupae of the Social Bees, a very curious 

 provision is made. There is a certain set, to which the name of Nurse- 

 bees has been given, whose duty it is to cluster over the cells in which 

 the Nymphs or Pupae are lying, and to communicate the heat to them, 

 which is developed by the energetic movements of their own bodies, and 

 especially by respiratory actions of extreme rapidity. The nurse-bees 

 begin to crowd upon the cells of the nymphs, about ten or twelve hours 

 before these last come forth as perfect Bees. The incubation (for so it 

 may be called) is very assiduously persevered in during this period by 

 the Nurse-bees ; when one quits its cell, another takes its place ; and 

 the rapidity of the respiratory movements increases, until they rise to 





