SOUTH AFRICAN BAGWORMS. 637 



•doptera expert, while a careful examination of the larvas and 

 pupse shows the differences at once. 



For these reasons alone the careful study and description 

 ol the larvae and pup;i3 are essential in the discussion of a 

 species ; but there is still another very important reason. 

 When we desire to determine the benefit derived from the 

 presence of certain insectivorous birds or other animals which 

 make, at certain times at least, insects a part of their diet, it 

 Ijecomes necessary to examine the stomachs or ejecta of such 

 birds or quadrupeds, and to scrutinise the chitinous remains 

 of any insect which may be found therein. It is evident that 

 only the more strongly chitinised parts will escape the digestive 

 and crushing action of stomach or gizzard, and we find in 

 general only head-cases, mouth-parts, or feet, to guide us in 

 our determination. Unless these parts have been carefully 

 tfigured and described we are at a loss to say with certainty 

 whether a certain injurious species is being preyed upon by 

 the alleged predaceous enemy. 



From a purely scientific standpoint the study of the imma- 

 •ture stages is equally important, as they will show characters 

 which may assist in clearing up doubtful points in the rela- 

 tionships of certain groups, relations which have been based 

 almost exclusively on the characters (wing venation) of the 

 -adults. The study of the position of the setse on the cater- 

 pillars has come much to the fore in this connection in recent 

 years, and has proved to be of great importance in establishing 

 such relationships. And here the study of the first instar of 

 the larva is of the greatest importance. 'I'he first instar of 

 the caterpillar is the most primitive condition, and at this 

 stage it is more apt to show its phylogenetic relationships 

 than in the succeeding instars. With the successive moults 

 numerous changes occur which are due to adaptation to the 

 ■environment, either to afford concealment, or to guard against 

 the attacks of parasites or predaceous enemies. No cases are 

 "known to the writer where larvae in the first instar have been 

 parasitised, while the smaller size of the newly-hatched larvae, 

 and the usually more or less concealed position of the eggs 



