CONCLUDING REMARKS ON THE ARTRROPODA. 221 



CONCLUDING REMARKS. 



We have thus arrived at the conclusion of a necessarily very condensed sketch of the vast group 

 of animals arranged by naturalists in the great division of the Arthropoda, a group which is certainly 

 the most numerous in species, and probably also in individuals, of all the great primary sections into 

 which the animal kingdom may be divided. A few additional words summing up the relations of the 

 oroup as a whole may not, however, be out of place. 



A consideration of the habits of the species and their relations to the world at large seems 

 to indicate that the primary function of the group in. general is that of a natural police, acting 

 sometimes openly in the sight of all, sometimes in a concealed fashion, which renders it difficult 

 to realise the extent of their influence. Among the scavengers both of land and water the foremost 

 place must certainly be assigned to Arthropods, numbers of which seem to be constantly on the 

 watch for all those articles called by the French " immondices," the continued presence of which, 

 either in the air or in the water, cannot fail to be either offensive or injurious to other living 

 organisms. Excrementitious matters and putrefying or decaying animal and vegetable substances are 

 thus rapidly got rid of and brought once more into the cycle of vitality, and in these useful operations 

 thousands of species of insects of different orders, many mites, and a very large proportion of the 

 class Crustacea, are perpetually engaged. 



Of the rest, while there are some which seem to have no particular mission, the great majority 

 may be regarded as acting more or less powerfully as checks upon the increase of other animals and 

 plants, and this often in so direct a manner that our best examples of the system by which the 

 numerical proportions of different kinds of organisms are maintained in the world are to be derived 

 from the study of these creatures. The whole series of predaceous insects, the carnivorous Myriopods, 

 and the great mass of the Arachnida, are most efficient agents in keeping down the development of 

 their weaker fellows, while a host of plant-eating species, especially of insects, perform the same part 

 for the vegetable kingdom. Parasitism, which is common throughout the three great classes of 

 Arthropoda, and manifests itself in many very remarkable ways, plays a most important part 

 in checking the increase of animals of many kinds, and, as we have seen, provides a peculiarly 

 delicate means of regulation, seeing that under the influence of parasites the creature affected is able 

 to perform its principal functions in the economy of nature, but is weakened or altogether destroyed 

 when the time of reproduction arrives. 



The action of the Arthropods in nature is in numerous cases greatly intensified by the important 

 changes through which so many of them pass in the course of their life-histoiy. Phenomena of more 

 or less similar character certainly occur in other groups, but those extra-ovular changes which 

 we dignify by the title of metamorphoses, and which in their extreme manifestations make one 

 animal play the part of two, constitute a general characteristic of the Arthropoda, and have a 

 most important bearing on their life-history. Of the metamorphosis we may distinguish two kinds in 

 the Arthropoda generally. In the great majority of the types distinguishable in the group, we find 

 what may be called a " direct " metamorphosis, that is to say, the young animal escapes from the egg 

 in a form differing more or less from that of its parents, but destined to reach the mature form by 

 simple growth and development of its parts with or without the addition of new parts as it advances 

 in age, a mode of development which we recognise throughout the Crustacea, Arachnida, and 

 Myriopoda, and in the whole of the lower (or hemimetabolous and ametabolous) insects. In the 

 metabolous insects, or insects with a complete metamorphosis, we find another set of phenomena 

 superadded, a more or less worm-like larva stage being intercalated between the egg and the 

 perfect insect. The explanation of this seems to be furnished by the life-history of certain parasitic 

 forms of Coleoptera, such as the Meloidse and Stylopidse, in which the insect when first hatched is a 

 little six-legged creature presenting all the external characters of a larva destined to undergo 

 direct development towards the perfect form, but subsequently giving origin to a soft, maggot-like 

 larva, which would never be supposed to have any connection with its predecessor. It seems 

 probable that, in the history of the class Insecta, a similar change, the traces of which are now 

 preserved only in a few species, may have taken place in the course of development of certain 

 forms, and that through these the whole series of insects with a complete metamorphosis may have 



