170 CONSIDERATIONS OF CLASSIFICATION AND METHODS 



again, certain insects are different from others in having scales on the 

 wings. We find that not only do the Culicinae have such character- 

 istics, but the same is observed with the Anophelinae and other similar 

 scale-wing insects. All of these we term a Family and we speak of the 

 Culicidae, meaning the family of mosquitoes. The name of a family 

 ends in "idae." Many families are not subdivided into subfamilies, 

 but are directly separated into genera. Again, a genus may have only 

 a single species. When there are a number of families agreeing 

 closely in some striking characteristic, we group them together into an 

 Order; thus, the family of mosquitoes closely resembling many other 

 families of insects in possessing a pair of well-developed wings are 

 grouped in the order Diptera; all of which resemble certain other 

 animals in the possession of a distinct head, thorax and abdomen with 

 3 pairs of legs projecting from the thorax. This collection of animals 

 we call a Class; thus, we speak of the class Insecta. It will be observed 

 that the insects have no internal skeleton, but instead a chitinous 

 cuticle, the exoskeleton. Spiders, ticks, etc., resemble them in this 

 respect, and we now apply to all such animals the wider designation, 

 Branch or Phylum. Inasmuch as the animal kingdom is divided into 

 the branches Protozoa, Porifera, Ccelenterata, Echinodermata, 

 Vermes, Arthropoda, Mollusca and Chordata, we see that the branch is 

 the largest grouping we employ. To descend in the scale we have 

 belonging to the branch, the classes; to the class, the orders; to the 

 order, the families; to the family, the subfamilies; to the subfamily, 

 the genera; to the genus, the species. Occasionally a species is 

 further divided into subspecies. 



There are certain terms employed in animal parasitology which it 

 is necessary to understand. Among these we shall refer to the follow- 

 ing: 



i. True Parasitism. By this is understood the condition where 

 the parasite does harm to the host, deriving all the benefit of the 

 association. A good example of this would be the hook-worm infecting 

 man or animals. 



2. ! *Mutualism. In such an association there is mutual benefit to 

 each party of the association. An instance of this would be the 

 presence of colon bacilli in the intestines. The bacillus is furnished 



