4 ENTOMOLOGY 



segmented. Body elongate, with fifteen distinct terga, and eleven or 

 twelve pairs of legs. Cerci well developed. Genital opening in the 

 third body segment. One pair of spiracles, opening on the head, under 

 the antennae. Small arthropods not more than five or six millimeters 

 in length. Example, Scolopendrella (Fig. 6) . 



Myrientomata. Terrestrial. Three regions: head, thorax and 

 abdomen. Head small, conical, prognathous. One pair of eye-like 

 spots. Antennae absent. Mouth parts suctorial. Mandibles and 

 maxillae attenuate, styliform, protrusible and retractile. Labium 

 attenuate. Body strongly elongate, fusiform, narrowing posteriorly, 

 fifteen-segmented in adults. Thorax distinct from abdomen; pro thorax 

 shorter than meso- or metathorax. Three pairs pf thoracic legs, and a 

 pair of vestigial legs on each of the first three abdominal segments. 

 Last four abdominal segments more or less retractile. Cerci absent. 

 Genital opening posterior. Male genitalia elongate, retractile, distally 

 bilobed, with a pair of slender, forceps-like appendages. Female appen- 

 dage short, with short forceps. Minute delicate arthropods, seldom 

 more than one millimeter in length. Example, Acerentomon (Fig. 8.) 



A single order, Protura, discovered and named by Silvestri, and 

 consisting of two families: Acerentomidae, without a tracheal system, 

 and Eosentomidae, with simple tracheae and two pairs of thoracic 

 spiracles. Protura, easily overlooked on account of their small size, are 

 doubtless widely distributed. At present twelve species are known from 

 Europe and twelve from the United States, all but one of our species 

 having been described by Dr. H. E. Ewing. 



Insecta (Hexapoda). Primarily terrestrial. Three distinct regions 

 head, thorax and abdomen. Head with a pair of compound eyes in 

 most adults, one pair of antennae and typically three pairs of mouth 

 parts mandibles, maxillae and labium besides which a hypopharynx, 

 or tongue, is present. In Apterygota a fourth pair of mouth parts is 

 associated with the hypopharynx. Thorax with a pair of legs on each 

 of its three segments and usually a pair of wings on each of the posterior 

 two segments; though there may be only one pair of wings (as in Diptera, 

 male Coccidae and male Strepsiptera) ; the prothorax never bears wings. 

 Abdomen typically with eleven segments and without legs, excepting in 

 some larvae (as those of Lepidoptera, Tenthredinidae and Panorpidae). 

 Stigmata paired and segmentally arranged. A metamorphosis (direct 

 or indirect) occurs except in Thysanura and Collembola. 



Relationships. The interrelationships of the classes of Arthropoda 

 form an obscure and highly debatable subject. 



