6o 
The Simplest Insects 
two groups or masses, one on each side of the body, in all other insects 
(Fig. 66), are separate and arranged segmentally in Japyx (Fig. 88), and 
less markedly so in Machilis; the respiratory system of Machilis (Fig. 89) 
consists of nine pairs of distinct, segmentally arranged groups of tracheae 
(air-tubes), while the ventral nerve-cord has a ganglion in almost every seg¬ 
ment of the body. As insects are certainly descended from ancestors whose 
bodies were composed of segments much less interdependent and coordi¬ 
nated than those of the average living insect, those present-day insects which 
have the body both externally and internally most strongly segmented are 
believed to be the most generalized or primitive of living forms. In addi¬ 
tion to the segmented character of the internal organs we have also another 
strong evidence of the primitiveness of the order in the possession by several 
Aptera of rudimentary but distinct external pairs of appendages on the 
abdominal segments, appendages undoubtedly homologous with the thoracic 
legs, and probably well developed in the insect ancestors as abdominal legs 
like those of the centipeds. 
The order Aptera is composed of two suborders, which may be dis¬ 
tinguished as follows: 
Abdomen -elongate, composed of ten segments, and bearing long bristle-like or 
shorter forceps-like appendages at its tip; no sucker on ventral side of first 
' abdominal segment; antennae many-segmented. Thysanura. 
Abdomen short and robust, composed of six segments, and usually with a forked 
spring at tip (usually folded underneath the body), and with a ventral sucker 
on first abdominal segment; antennae 4- to 8-segmented. Collembola. 
Thysanura.— This suborder includes three families (a problematical 
fourth family is found in Europe), as follows: 
Body covered with scales. Lepismid^e 
Body not covered with scales. 
Tip of abdomen with forceps-like appendages. Japygid^e. 
Tip of abdomen with slender many-segmented appendages. Campodeid^e. 
To the last family in the above key belongs the interesting creature 
Campodea stapliylinus (Fig. 90), which zoologists regard as the most primi¬ 
tive living insect. It is small, white, flattened, wingless, and so soft-bodied 
and delicate that it can hardly be picked up uninjured with the most deli¬ 
cate forceps. It is about \ inch long (exclusive of caudal appendages), and 
is to be looked for under stones and bits of wood. I have found it in Ger¬ 
many, in New York, and in California, which indicates its wide distribu¬ 
tion. Other collectors have taken it in Italy, England, and in the Pyrenees. 
It is said to live also in East India. Is it not a little surprising that this 
most primitive, wholly defenceless, and ancient insect should be able to live 
successfully the world over in the face of, and presumably in competition 
with, thousands of highly developed specialized modern insect forms? It 
