94 NATURAL HISTORY OF ARTHROPODS, 



the blood. Here, as in the higher vertebrates, respiration raises the temperature of 

 the body, and for the same reason. We have the authority of both Huber and New- 

 port for the fact that humble-bees when incubating pupae raise the temperature by 

 increasing the number of respirations. 



We can best discuss the question of the development of insects, and especially the 

 metamorphoses, in connection with the various groups, there being such differences 

 that a general account which attempted to take cognizance of all would be very con- 

 fusing. There are, however, several features which are common to all which may be 

 mentioned here, especially since they serve to make more evident the great differences 

 existing between the Crustacea and the Insecta. 



The segmentation of the egg is iisually superficial, the central portion not dividing 

 at first. From this central yolk arises the primitive stomach of the embryo, a 

 marked difference from the way in which the same portion is formed in the crustaceans, 

 where, as we have seen, there is a true invagination. At an early stage the eggs of 

 most insects become enveloped in a cellular membrane, which, from being formed in 

 a strikingly similar method to that in which one of the foetal membranes of the higher 

 vertebrates arises, has been called the amnion. The appendages grow in much the 

 same way as in the group just passed, but it is to be noticed that in no stage of the 

 development of the insects do we find a two-branched appendage, a feature so common 

 among the crustaceans. The middle germinal layer (mesoblast) also arises in very 

 distinct ways in the two groups. 



There is no group of animals upon which more has been written than upon the 

 insects. Ever since naturalists began the study of nature these forms have attracted 

 especial attention. From every point of view they possess interest. Their shapes and 

 colors make them attractive to lovers of the beautiful and the grotesque ; their habits 

 are interesting, and their metamorphoses are marvellous. There is a far more practical 

 side to their study. A large number are of economic importance. Some few are of 

 direct value to man : from the silk-worm we obtain one of the most valuable textile 

 materials, from the cochineal insect one of our most brilliant dyes. In another very 

 important manner insects play a part in matters which affect human interests. It is 

 now, thanks to the labors of Darwin and Hermann Muller, a well-known fact that a 

 large proportion of the flowering-plants are incapable of self-fertilization, and were not 

 pollen brought to the stigma from another flower of the same species no seeds would 

 be produced. This fertilization is effected to a certain extent by the winds and other 

 agencies of like character, but it is to the insects that we must turn for the most effec- 



O 7 



tive Avork in this line. These in their search for honey visit plant after plant, and 

 from one they carry the pollen which becomes entangled upon their legs or bodies, 

 and in such a position that in the next flower visited it will be brought in contact with 

 the stigma, and fertilization will thus be effected. It has been clearly shown that for 

 this purpose there exist many mutual adaptations, there being many insects which 

 can fertilize only one species of plant, and, conversely, many flowers which require 

 the presence of a peculiar insect to carry the pollen from the stamens to the pistil. 

 The subject is a large one, and we can but touch it in this brief manner; but those 

 who wish to study it further will find ample material in the writings of Darwin, 

 Muller, and Trelease, and better, in the relations of the animals and plants around 

 them. 



Still other insects are of value to man from their carnivorous habits. Some of these 

 dispose of large amounts of refuse matter which would otherwise decay, producing 



