THE LARVA 35 



round holes, often inclosed in a ring or a patch of some prominent 

 colour. These are the spiracles or openings of a series of air tubes 

 called trochees. These latter divide and subdivide within the body 

 of the caterpillar, the branches of one often uniting with those of 

 another, thus forming a really complicated arrangement of air pipes 

 by which the supply of oxygen is distributed. 



A microscopic examination of a portion of one of the tracheae 

 will show that its walls are supported by an elastic spiral of a firm 

 substance. This arrangement serves to keep the air passages open, 

 and secures for the caterpillar a free supply of air at times when a 

 contraction of the segments would otherwise cause the tubes to 

 collapse. 



There are nine spiracles on each side of the caterpillar's body, 

 and never more than one in the side of the same segment. The 

 head, which we have been regarding as the first segment, has no 

 spiracles. The second segment has a pair one on each side. 

 There are none in the third and fourth ; but all the segments, from 

 the fifth to the twelfth inclusive, have each a pair ; the last 

 (thirteenth) segment has none. 



We have already observed the general arrangement of the cater- 

 pillar's limbs ; but perhaps it may be interesting and even con- 

 venient to the reader to give here a little table that will show at a 

 glance the disposition of both limbs and spiracles. 



First segment head. . . Two short antennas, two jaws, 



and twelve eyes. 



Second . . > . Legs and spiracles. 

 Third Legs only. 



Fourth . . . Legs only. 



Fifth . . . Spiracles only. 



Sixth Spiracles only. 



Seventh ,, . . . Spiracles, and sometimes 



claspers. 

 Eighth . . . Spiracles, and sometimes 



claspers. 

 Ninth Spiracles, and sometimes 



claspers. 

 Tenth . . . Spiracles, and generally 



claspers. 

 Eleventh ,, . . . Spiracles only. 



Twelfth , Spiracles only. 



Thirteenth Claspers only, and these 



occasionally absent. 



