120 HOUSE, GARDEN, AND FIELD 



beneath the abdomen are as yet undeveloped. Until its 

 first moult the larva has on the top of its head a tooth, 

 which was used to break through the egg-shell. Earwigs, 

 fleas and other insects, besides some centipedes, have a 

 tooth or spine on the top of the head for breaking the 

 egg-shell. A similar contrivance is found on the tip of 

 the snout in reptiles, birds, and egg-laying mammals. 



Silver fishes never acquire wings, nor pass through a 

 resting stage, though they change the skin several times. 

 From the first they are very like their parents, and after 

 the first moult, which takes place seven days after hatch- 

 ing, they differ in no important respect. 



Another small insect very like a silver fish may often 

 be found in fields and gardens by a close observer. This 

 is called Campodea. It is still smaller than the silver 

 fish, never exceeding one quarter of an inch in length, and 

 has a white skin. It is so delicate that it cannot be picked 

 up without injury, and it dies as soon as it is placed in a 

 tube or bottle. Campodea has only two tails, the middle 

 one being absent. Like a silver fish it goes through no 

 transformation. A third creature of the same order 

 (Thysanura) lurks about bakers' ovens, while a fourth is 

 sometimes very plentiful on rocks near the sea. 



Many carnivorous beetle-larvae are strikingly similar to a 

 silver fish or a Campodea, having a slender body, long legs, 

 long antennae, and two or three tails. Such larvae occur 

 more or less frequently in all the lower orders of insects, 

 and may be said to be the ordinary larvae of these groups. 

 The silver fishes are believed by naturalists to resemble 

 the most ancient of all insects, for there is reason to believe 

 that the first insects which ever appeared on the earth had 

 a simple life-history, and never possessed wings at all, re- 

 sembling in these respects such Arthropods as centipedes and 

 millipedes. When wings were first acquired by insects, they 

 were probably developed rather late in life, and not all at 

 once, but by degrees, growing larger and larger at each 

 moult, as they still do in some insects. The acquisition of 



