74 ANIMAL PARASITES. 



bourhood of the neck. These insects are called Ametabola, i. e., 

 insects without a metamorphosis for example, the lice. 



2. In a second case, we certainly find the external form pretty 

 like that of the parents, but the young animals are still destitute 

 of some organs of the adult, especially the wings. These semi- 

 larvse, as they are called, only become similar to their parents by 

 moulting. The state in which only immoveable wing-sheaths 

 and no moveable wings are presented, has been sometimes called 

 the pupa. It is essential that these semi-larva? as pupae always 

 eat and move about. These are the Insect a hemimetdbola, or those 

 with an imperfect metamorphosis for example, the bugs. 



3, In the third case the young, on their exclusion from the egg, 

 are quite unlike their parents, and undergo a complete metamor- 

 phosis through three different, well-defined conditions. 



a. Larvae, which eat, grow, and usually change their skins 

 several times. They have the form of jointed worms ; are foot- 

 less, or furnished with six legs, and sometimes also with false feet ; 

 are very soft, sometimes smooth, sometimes with hairs, spines, 

 horns, or lateral appendages, which assist in walking, or as tufts 

 of bristles in swimming. The head is sometimes soft, sometimes 

 horny, and bears the oral organs, which are frequently much re- 

 duced. Manducatory organs are always present, sucking organs 

 only in the headless larvse of flies and some Hymenoptera, which 

 are parasitic in the larva state. In herbivorous larva? the jaws 

 are broad and toothed within ; in carnivorous larvse they are hook- 

 shaped, and often perforated to their apex, so that the nourishment 

 is taken up by this instead of the mouth, which is deficient ; the 

 upper lip (labrum) is usually wanting ; the maxillse are usually 

 present, but only conical and without lobes. The lower lip 

 (labium) is greatly developed in those which spin themselves up, 

 and bears the opening of the silk-glands. The antennae are retro- 

 grade or entirely wanting ; the palpi usually small, conical, and 

 two-jointed. The eyes are few and simple, never compound, and 

 sometimes entirely deficient. 



The intestinal canal is always large and wide, especially in 

 herbivorous species, filling the entire cavity of the body ; the chyle- 

 stomach is especially large ; the ileus small. The species which 

 spin a cocoon have two long tubes (silk-glands), with a sticky 

 secretion which immediately hardens into threads in the air. 

 Every cocoon consists only of a single thread. The dorsal vessel 

 is greatly developed ; the individual ganglia of the ventral cord 



