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 
hmbs 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 antenne have become straightened and greatly elongated, 
and a corresponding change has occurred in the maxille and 
Jabium 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 
mnetathoracic 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 species*. Researches on the 
embryology of the higher Annelids show that the development 
of worms proceeds from a single centre ty. 
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. 1. p. 22. 
Tt See S. Lovén, K. Vetenskaps-Acad. Hand]. 1840 (Wiegmann’s 
Archiv, 1842, part 1). M. Sars, Development of Polynoé cirrata (Wieg- 
mann’s Avahive 1845, part 1). Milne-Edwards (Ann. Sc. Nat. 1845). 
Ann. & Mag. N. Hist. Ser.3. Vol. xviii. 7 
