6 ON THE ANATOMY OF THE FLY. 



especially in the organs of special sense, end in nerve cells. The nerves 

 of the ganglia of the head and thorax are rounded, the fibres of which 

 they consist being enclosed in a neurilemma; but those of the sympathetic 

 system appear to have no neurilemma : they spread out into flattened 

 branched expansions, and have numerous branching nerve cells scattered 

 amongst them. 



Section II. Development. 



Perhaps the most startling difference between the vertebrate 

 and the articulate divisions of the animal kingdom is seen in 

 the manner of their development. In insects, as in many other 

 articulata, this takes place by a process termed Metamorphosis, 

 the creature not only being entirely and suddenly changed in ap- 

 pearance, but also in habits of life. Usually there are three 

 periods in the existence of an insect, the first characterised by rapid 

 growth and a comparatively small exercise of animal functions, 

 the second by a period of apparent quiescence, during which 

 growth is changed into development, and lastly one in which 

 the animal functions are seen in their highest perfection ; these 

 are called respectively the larval, pupal, and imaginal states. 



In the larval state, where growth is rapid, the hard in- 

 tegument is usually shed periodically, its place being supplied by 

 new layers deposited beneath it, just as the cuticle in vertebrates is 

 renewed by younger epithelial cells, or as the old bark of trees 

 is replaced by younger layers. The larva of the fly forms an ex- 

 ception and does not shed its skin. After the full growth is 

 attained, the insect commonly assumes a different form, called a 

 pupa or chrysalis when it becomes inactive, and a nymph when a 

 greater or less amount of activity remains. In a certain number 

 of days the pupa case bursts, or the nymph sheds its skin, and the 

 perfect insect emerges, partially developed at first, but in a few 

 minutes or hours, as the case may be, assuming its new functions 



