INSECTS AFFECTING PARK AND WOODLAND TREES 17 



marked increase in size usually indicates a molt. Ur II. ij. Dyar' has shown 

 that a nearly constant ratio exists between the widths of the head of a cater- 

 pillar in its various stages. It is interesting to note in this connection that 

 some species of female lepidopterous larvae have an additional molt. 

 Ecdysis in forms having an incomjilcte metamorphosis is practically the 

 same as that described for a caterpillar except that it is usually accompanied 

 by a greater change in form or a nearer approach to the external appear- 

 ance of the adult. 



The larvae belonging to one order present many diversities of structure 

 but they may all be considered as developments from a more simple or gen- 

 eralized form. A comparison of those belonging to various orders will 

 reveal certain analogies and it will be seen that those less highly developed 

 possess many features in common with the lowest or simplest insects, the 

 Thysanura. They all have, generally speaking, a distinct head, well devel- 

 oped mouth parts, simple eyes, 6 thoracic legs and an abdomen destitute of 

 true legs. The membranous prolegs of hymenopterous, coleopterous and 

 lepidopterous larvae are secondary developments to meet the necessities of 

 existence and vary in number from a large anal one among many beetles to 

 1 6 in some sawflies. The same is also true of other structures, such as 

 spines, hairs and tul^ercles, for while these modify the appearance of the 

 bearer very much, they are of secondary importance. 



The larvae of some of the more highly developed insects, such as bees 

 and flies, are legless, and have a very small head and poorly developed 

 mouth parts. The explanation of this may be found in the fact that these 

 larvae are not compelled to make any effort to obtain food. The young of 

 bees are fed by their parents and those of flesh flies and of many parasites 

 find themselves surrounded by sustenance. The same is true of species 

 living in decaying vegetable matter or in many vegetable galls. One of 

 nature's laws is that useless organs must eventually disappear and the 

 degraded or apparently undeveloped condition of these larvae must be 

 regarded as the result of their manner of living. Bearing in mind this law, 

 ■Psyche. 1890. 5:420. 



