Knab's article should serve a valuable end in checking irresponsible theorizing on the subject of insect transmission of disease. Nevertheless, the principles which he laid down cannot be applied to the cases of accidental carriage of bacterial diseases, or to those of direct inoculation of pyogenic organisms, or of blood parasites such as the bacillus of anthrax, or of bubonic plague. Accumulated evidence has justified the conclusion that certain trypanosomes pathogenic to man are harbored by wild mammals, and so form an exception. Townsend believes that lizards constitute the natural reservoir of verruga; and it seems probable that field mice harbor the organism of tsutsugamushi disease. Such instances are likely to accumulate as our knowledge of the relation of arthropods to disease broadens.
CHAPTER XII
HOMINOXIOUS ARTHROPODS
The following synoptic tables are presented in the hope that they may be of service in giving the reader a perspective of the relationships of the Arthropoda in general and enabling him to identify the more important species which have been found noxious to man. Though applicable chiefly to the arthropods found in the United States, exotic genera and species which are concerned in the transmission of disease are also included. For this reason the keys to the genera of the Muscids of the world are given. As will be seen, the tables embrace a number of groups of species which are not injurious. This was found necessary in order that the student might not be lead to an erroneous determination which would result were he to attempt to identify a species which heretofore had not been considered noxious, by means of a key containing only the noxious forms. The names printed in bold faced type indicate the hominoxious arthropods which have been most commonly mentioned in literature.
CRUSTACEA
Arthropods having two pairs of antennæ which are sometimes modified for grasping, and usually with more than five pairs of legs. With but few exceptions they are aquatic creatures. Representatives are: Crabs, lobsters, shrimps, crayfish, water-fleas, and woodlice. To this class belongs the Cyclops ([fig. 122]) a genus of minute aquatic crustaceans of which at least one species harbors Dracunculus medinensis, the Guinea worm ([fig. 121]).
MYRIAPODA
Elongate, usually vermiform, wingless, terrestrial creatures having one pair of antennæ, legs attached to each of the many intermediate body segments. This group is divided into two sections, now usually given class rank: the Diplopoda or millipedes ([fig. 13]), commonly known as thousand legs, characterized by having two pairs of legs attached to each intermediate body segment, and the Chilopoda or centipedes ([fig. 14]) having only one pair of legs to each body segment.