CLASSIFICATION 9 
part of entomologists has been followed by a gradual increase 
in the number of orders, until our present system has been 
attained. 
Owang to the incomplete condition cate 
of entomological knowledge, however, 
the best system as yet proposed is but 
tentative and more or less open to 
objection. The most competent and 
widely approved classifications are 
those of Brauer and Packard, and 
the system here adopted is essentially 
that of Brauer, with certain important 
modifications made by Packard. 
In the course of the following syn- 
opsis of the orders of insects it is 
necessary to use some terms, as mcta- 
morphosis and thysanuriform, in an- 
ticipation of their subsequent defini- 
tion. 
1. Thysanura.—No metamorphosis. 
Mouth parts mandibulate, either free The snow flea, Acho- 
rutes niwicola. Length, 
(ectopnathous) or enclosed. in the 2 mm. 
head (entognathous). Wings inva- 
riably absent. Thoracic segments simple and similar. Ab- 
dominal segments ten, 
Fig. 12. ; : : 
with two to eight pairs 
of rudimentary limbs 
and two or three anal 
cerci. Eyes aggregate, 
compound or absent. 
Antennz multiarticulate. 
Integument thin. Ex; 
amples, Campodea (Fig. 
9), Japyx, Machilis, 
Lepisma (Fig. 10). Some one hundred and seventy-five spe- 
cies are known. 
Sminthurus hortensis. Length, 1.2 mm. 
