282 Journal New York Entomological Society, t'^'o'- xxiv. 



filament, varies from fifteen to twenty-two millimeters in length. 

 This insect resembles Lepisma and Machilis morphologically, but 

 differs from them in many respects chief among which is the occur- 

 rence of but a single caudal filament. This group contains the family 

 Dasyleptidse. Although here ranked as an order, the group (which 

 is related to the Machilidas) may possibly be of the value of suborder 

 only; but this point can be determined only upon a more detailed 

 study than has been given the subject by Brongniart. 



8. Machiloides (Trinemura). — Three caudal filaments (a 

 median unpaired terminal filament, and a pair of lateral filaments 

 homologous with the cerci) occur in this group. Head usually 

 directed downward (hypognathous), not flattened dorso-ventrally, 

 but laterally compressed, in frontal view deeper than broad. Eyes 

 usually large, and extending forward above the antennae are ap- 

 proximate or almost contiguous above. They are composed of rela- 

 tively numerous small ommatidia. Mouthparts of the exposed type: 

 maxillary palpi usually seven-segmented. Pronotum usually shorter 

 than mesonotum. Thoracic sterna not in the form of broad plates 

 overlapping the coxee somewhat laterally (as in the next group to be 

 considered). Coxae subcylihdrical : those of the meso- and meta- 

 thorax usually provided with styliform appendages. Tarsi usually 

 three-segmented. Abdomen composed of eleven segments, the ter- 

 gum of the tenth not partially covering the eleventh. Body sub- 

 cylindrical, tapering posteriorly, and is usually clothed with scales. 

 Insects usually capable of springing. Abdominal ganglia eight in 

 number. Seven pairs of spiracles in the abdominal region. The 

 tracheae of one side of the body do not anastomose with those of the 

 other side nor are those of the same side connected by longitudinal 

 trunks, the tracheae being arranged segmentally. There are twelve 

 malpighian tubules. Ovaries wtih seven ovarioles. Heart with nine 

 pairs of ostia. This group contains the family Machilidae. 



9. Lepismatoides (Thysanura). — Two segmented cerci, in addi- 

 tion to the median unpaired terminal caudal filament, occur in this 

 group. The unpaired median filament according to Heymons, is 

 merely a prolongation of the body which has become annulated, and 

 is therefore not strictly comparable to the paired filaments, which 

 are homologous with cerci. In this group, the head is usually directed 

 forward (prognathous) and is flattened dorso-ventrally, in frontal 



