RELATIONSHIPS 323 



mitted by arthropods but to be exclusively transmitted by certain 

 species or genera. In the latter category, as far as we know at 

 present, are malaria, by some physicians rated as the most im- 

 portant human disease; typhus fever, the unseen dragon of death 

 which hovers over every war camp in the world; yellow fever, 

 which formerly haunted South and Central America; sleeping 

 sickness, the scourge of Central Africa; Chagas' disease of 

 South America; relapsing fever; Rocky Mountain spotted fever; 

 dengue; phlebotomus fever; Japanese flood fever; filarial dis- 

 eases; guinea-worm infection; lung fluke infection; some tape- 

 worm infections; and others of less importance. Some other 

 important diseases, such as kala-azar and oroya fever, are be- 

 lieved to be transmitted by arthropods but the transmitting 

 agents have not yet been discovered. 



There are many other diseases which, although they may be 

 transmitted in other ways also, are commonly disseminated by 

 insects, often in a more or less mechanical way. Such are plague, 

 tuberculosis, leprosy and others. In the case of some of these 

 diseases, e.g., plague, the intestines of the transmitting arthro- 

 pods serve as culture tubes for the disease germs, whereas in 

 other cases, e.g., amebic dysentery, the arthropods are merely 

 passive carriers of disease germs which adhere to their feet or 

 bodies. It is evident that any insect may serve as a disseminator 

 of disease in this mechanical way in direct proportion to the ex- 

 tent that it associates with man and that its habits bring it in 

 contact with disease germs. 



Relationships. — The insects and their allies, constituting the 

 phylum Arthropoda, are the most highly organized of inverte- 

 brate animals, and stand at the head of their particular line of 

 evolution. They find their nearest allies in the segmented worms 

 or annelids, i.e., earthworms, leeches, etc., but most of them show 

 a great advance over their lowly cousins. Like the annelids 

 they have a segmented type of body, though in some types, such 

 as the mites, all the segments become secondarily confluent. 

 Like the annelids, also, the arthropods are protected by an ex- 

 ternal skeleton which usually consists of a series of horny rings 

 encircling the body. The most obvious distinguishing character- 

 istic of the arthropods is the presence of jointed appendages in 

 the form of legs, mouthparts and antennae. Internally they are 

 distinguished from other invertebrates in that the body cavity, 



