20 THE BUTTERFLIES OF THE 



divide and subdivide till the minute tubes penetrate all 

 parts of the body, especially all parts of the circulatory 

 system. It is in these ultimate divisions that the inter- 

 change of gases takes place which constitutes the purifi- 

 cation of the blood, or circulating fluid. 



The head is of a rounded or oval form, and has a harder 

 covering than the other parts of the body. When the 

 larva is first hatched the head is nearly globular, divided 

 down the front by a suture which forks about the middle. 

 As the larva approaches maturity the head usually 

 changes in shape, assuming the characters that are 

 peculiar to the species. The lower edge of the little 

 triangular piece which stands between the forks of the 

 frontal suture supports a little membrane, the labrum, 



Fig. 3. 



a, Head of larva of Danais Arcliippus from beneath, y^lQ: Jb, labrum ; md, inaudi- 

 ble; mr, maxilla, with two palpi; Via, labium, with one pair of palpi; s, spinneret; 

 a, antennie ; o, ocelli ; 6, side view, and c, front view, X 3. (Scudder, after Burgess.) 



or upper lip, and back of this are two stout biting jaws, 

 or mandibles, with serrated edges, that work laterally (see 

 Fig. 3). The mouth lies between these jaws, and back of 

 them are the secondary jaws, or maxillse, which in many 

 insects have a movement similar to that of the mandibles, 

 but they do not in the butterfly larvae. They consist 

 of a pair of fleshy prominences, and each of them has 



