METAMORPHOSIS I 6 I 



Some Insects take the same kind of food throughout their lives, 

 but many others change totally in this respect, and their organs 

 for the prehension and digestion of food undergo a corre- 

 sponding change. Butterflies suck food in the form of liquid 

 juices from flowers by means of a delicate and long proboscis, 

 while the young butterfly the caterpillar disdains sweets, 

 and consumes, by the assistance of powerful mandibles, a great 

 bulk of leaves. Other Holometabola undergo no such total change 

 of habits ; the tiger-beetle, for instance, is as ferocious a con- 

 sumer of the juices of Insects in its young stage as it is in the 

 adult condition. Hence Brauer l divides Insects, as regards this 

 point, into three categories. The forms in which both the young 

 and adult take food by suction he calls Menorhyncha ; those in 

 which both the imago and immature forms feed by mandibles he 

 calls Menognatha ; while his Metagnatha consists of those insects 

 that take food by jaws when young, but by suction with tubular 

 mouths when mature. Besides these main divisions there are 

 some exceptional cases to which we need not here allude, our 

 present object being to indicate that in the Metagnatha the 

 digestive organs are of a very different nature in the young and 

 in the adult states of existence. 



The internal organs for the continuance of the species are 

 known to be present in a rudimentary stage in the embryo, and 

 it is a rule that they do not attain their full development until 

 growth has been completed ; to this rule there may possibly be- 

 an exception in the case of the Aptera. But little information 

 of a comparative character exists as to the dorsal vessel and the 

 changes it undergoes during metamorphosis. There is con- 

 siderable difficulty in connexion with the examination of this 

 structure, but it appears probable that it is one of the organs 

 that changes the least during the process of metamorphosis. 



The exact nature of the internal changes that occur during 

 metamorphosis is almost a modern subject. It is of course a 

 matter of great difficulty to observe and record changes that go 

 on in the interior of such small creatures as Insects, and when 

 the phenomena occur with great rapidity, as is frequently the 

 case in Insect metamorphosis, the difficulty is much increased. 

 Nevertheless the subject is of such great interest that it has been 

 investigated with a skill and perseverance that call for the 



1 "Syst. Zool. Stud." S. Ak. Wien, Abth. 1, xci. 1885, p. 291. 

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