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. ARACHNIDA In this class the antennæ are apparently wanting, wings are never present, and the adults are usually provided with four pairs of legs. Scorpions, harvest-men, spiders, mites, etc. HEXAPODA (Insects) True insects have a single pair of antennæ, which is rarely vestigial, and usually one or two pairs of wings in the adult stage. Familiar examples are cockroaches, crickets, grasshoppers, bugs, dragon-flies, butterflies, moths, mosquitoes, flies, beetles, ants, bees and wasps. ORDERS OF THE ARACHNIDA a. Abdomen distinctly segmented. A group of orders including scorpions, (fig. 11), whip-scorpions (fig. 10), pseudo-scorpions, solpugids (fig. 12) harvest-men (daddy-long-legs or harvestmen), etc. ARTHROGASTRA aa. Abdomen unsegmented, though sometimes with numerous annulations SPHÆROGASTRA b. A constriction between cephalothorax and abdomen (fig. 7). True Spiders ARANEIDA bb. No deep constriction between these parts.