ARTHROPODA. 303 



latter is useful to man owing to the fact that it preys on certain hurtful 

 insects. In California the cottony-cushion scale, which in some way had 

 been imported from Australia, promised at one time to destroy totally 

 the orange industry. The Australian ladybug, which keeps it within bounds 

 in its native home, was imported, and the increase of the ladybugs was 

 such that the cushion-scales were all but destroyed. This species of ladybug 

 feeds exclusively on the cottony-cushion scale, and therefore the destruc- 

 tion of the latter led in turn to the rapid decline of the ladybugs from 

 the loss of their food supply. Indeed it was necessary to keep colonies 

 of the scale insects protected in order to furnish food and to prevent the 

 entire destruction of the imported beetle by starvation. In Australia where 

 both are at home the natural conditions and the adjustment of the two 

 species are such that this scale-insect does not become a pest. The dis- 

 covery of the biological relations of these species, and the relief of the 

 orange industry furnish a sample of the excellent work being done by the 

 U. S. Department of Agriculture in connection with the economic aspects 

 of biology. 



Order Hymenoptera (Membrane-wings). Hexapoda with four mem- 

 branous wings; mouth appendages adapted for sucking or for biting; 

 metamorphosis complete. This is the most -highly developed division of 

 Insecta, and embraces such 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. Some of the larvae are leaf-eating, as the rose- 

 slug, and others produce galls on the oak and other platits in depositing 

 their eggs. These are harmful to human interests. 



Class V. Arachnida (Spiders, Scorpions, etc.). Arachnida are arthro- 

 pods 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 confined to the abdomen, and may be of three 

 types : book-gills, associated with appendages (king-crab) ; trachea 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 

 that of the Crustacea, with which it was formerly classified. Numerous 

 related forms flourished earlier in the world's history but are now extinct. 



Order 2. Scorpionida (Scorpions'). Arachnids with a much elongated 1 

 and segmented abdominal region closely connected with the thorax. They 

 are air-breathers, with four pairs of book-lungs in the abdomen. The 

 posterior abdominal segments form a tail the last segment of which bears 

 a sting. See Fig. 148. 



