and Position of the Hymenoptera. 89 



far back, to the middle of the body, from their previous situation 

 very near the head and on the anterior third of the body. The 

 limbs are greatly enlarged ; the tarsi of the hind pair now 

 reach near the tip of the abdomen, whereas before they were 

 simply folded upon the thorax, not reaching to or resting upon 

 the abdomen. 



Great changes have occurred in the appendages of the head. 

 The clypeus, labrum, and mandibles are now exposed to view. 

 The antennae have become straightened and greatly elongated, 

 and a corresponding change has occurred in the maxillae and 

 labium with its palpi, which now reach to the middle of the 

 abdomen, while the lingua extends as far as the seventh abdo- 

 minal segment. This stage, therefore, is characterized by im- 

 portant modifications in the size and position of the extremities 

 and appendages of the head, thorax, and abdomen. In the 

 thorax the changes are not especially remarkable. The scutel- 

 lum is now in contact with the base of the abdomen, as if the 

 whole thorax had been carried backward, and the entire abdomen 

 brought forwards and upwards, due to the absorption of the 

 metathoracic ring and basal ring of the abdomen. 



Thus each of the three regions of the body is a centre of de- 

 velopment, the gradual perfection of the appendages belonging 

 to each region proceeding from the centre towards the periphery, 

 beginning at the insertion of the limbs to the trunk, and gra- 

 dually perfecting their development towards the extremity. 

 Hence the wings, the tarsi, or terminal joints of the limbs, and 

 the abdominal appendages are the last to be developed and 

 perfected. The anterior part of the thorax is perfected earlier 

 than the posterior, while in the abdomen the development goes 

 on from behind forwards. Prof. Dana has shown that in the 

 Crustacea the cephalothorax and abdomen are each a distinct 

 centre of development, in which progress reaches to a wider or 

 narrower circumference in different s{)ecies*. Researches on the 

 embryology of the higher Annelids show that the development 

 of worms proceeds from a single centre f- 



At this stage, which may be properly called the pupa state, 

 the eyes begin to turn dark, and a few hairs develope themselves 

 upon the upperside of the abdomen ; but the stage is so transi- 

 tory, that in a long series of individuals it is impossible to select 

 a single individual and denominate it a pupa, since there is no 



* Introduction to the Crustacea of the U. S. Exploring Expedition, 

 vol. i. p. 22. 



t See S. Loven, K. Vetenskaps-Acad. Handl. 1840 (Wiegmann's 

 Archiv, 1842, part 1). M. Sars, Development of Polyno'e cirrata (Wieg- 

 mann's Archiv, 1845, part 1). Milne-Edwards (Ann. Sc. Nat. 1845). 



Ann. ^ Mag. N. Hist. Ser. 3. Vol. xviii. 7 



