INSECTS AND THEIR NEAR RELATIVES. 
45 
unites developed in the gall can escape, in order to start new 
galls. In addition to the swelling of both surfaces of the 
leaf its internal structure is seen to be modified. In some 
parts there is a great multiplication of the cells, and in 
others a large part of the cells have been destroyed. Two 
eggs of mites are represented in this gall. As the season 
advances, and the galls become dry and brownish or black, 
the thickening of the leaf becomes less marked. In fact, in 
some cases there is a shrinkage of the parts affected. Fig- 
Fig. 54.—Section of leaf showing structure of gall in autumn : g, gall ; n , n , 
uninjured part of leaf ; o, opening of gall. 
ure 54 represents a section through a leaf collected and 
studied in October. 
Among the scavenger mites there are some that infest 
food products. Thus mites are sometimes found in cheese, 
in sugar, and in preserved meats. 
Class MYRIAPODA (Myr-iap'o-da). 
The Centipedes and the Millipedes. 
The members of this class are air-breathing Arthropods , in 
which the head is distinct from the thorax, and the thorax 
and abdomen form a continuous region, with from six to two 
hundred segments, each bearing a pair of legs. The head 
bears a single pair of antennce. 
The thousand-legged worms, as they are commonly 
called, are well-known and generally feared creatures. But 
few students find them attractive subjects of study; never¬ 
theless it is well to know something about them, for some of 
them are dangerous animals, and some are harmless. A few 
species are injurious to agriculture, while others are to be 
