ARTHROPODA 



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forms as bees, wasps, and ants. The most important habits of the group, which 

 are those growing out of their social life, have been referred to in the chapter on 

 adaptations (Chapter VIII). The chief economic value of the order is in the 

 honey of the honey-bee, the fertilization by bees of certain plants, as clover, and 

 the reduction of more hurtful species of insects by certain parasitic members of 

 the order, as the ichneumon-flies and chalcid-flies. Some of the larvs are leaf- 

 eating, as the rose-slug, and others produce galls on the oak and other plants in 

 depositing their eggs. These are harmful to human interests. 



Fig. 149. 



Fig. 149. Hornet's nest, sectioned. Photograph from life by Shufeldt. 



Class V. Arachnida (spiders, scorpions, etc.). — Arachnida are arthropods in 

 which the head and thorax are typically fused and represent about seven segments 

 with six pairs of appendages. There are no antennae. The abdomen is often 

 segmented but usually without paired appendages. Respiratory organs are con- 

 fined to the abdomen, and may be of three types: book-gills, associated with 

 appendages (king-crab) ; trachex similar to those of insects; and book-lungs (spiders). 

 Development is usually direct. 



Order I. Xiphosura (the king crab). — This order contains only one genus, 

 Limulus, a marine form with book-gills, and a cuticular test like thatof the Crus- 



