SOME OF THE RELATIONSHIPS between insects and plants have become so complicated that in most instances neither insect nor plant could long exist apart.

Similar relationships exist between insects and other animals and between certain insects and others of their own kind, yet their basic existence depends on the vegetable kingdom. Although millions of insects derive their subsistence from plants, they seem not to interfere greatly with the natural development of the plant world. It is true that in special situations insects might even exterminate a species of plant in a given location, and we know too well that they are responsible for tremendous losses to crops almost everywhere. However, it appears that plants have actually occupied as much of the earth's surface as is possible despite their insect dependents.

The ability to create a specialized type and supply of food is not uncommon among many groups of insects. Such modifications may affect other animal and especially insect hosts, as in the case of parasites, but are more often conspicuous in plant hosts. Chief among these latter are galls, produced by the dipterous gall midges or gall flies belonging to the family Itonididae, and the gall wasps of the hymenopterous family Cynipidae. Members of other orders, including certain species of thrips, lace bugs, psyllids, aphids, coccids, beetles, weevils, sawflies, trypetid flies, and possibly still others also produce plant galls in which the larvae develop. The galls formed by these various insects may also be inhabited by many other kinds of insects that feed upon the gall itself and by predators and parasites that prey upon all the various insects associated with the galls. Complicated biological relationships thus are associated with insect galls. The development of galls of different insects may vary somewhat, but generally it appears to be caused by excretions of the developing larvae or nymphs and to follow a more or less definite pattern for a given species or group of closely related species. Thus, the shape, vestiture, sculpturing, and color may be characteristic of a species or variety and may thus aid in recognizing them.

The galls of a Chinese aphid, Melaphis chinensis, are artificially reared on Rhus semialata in China in commercial quantities as a source of dye and tannin and for medical purposes. The host plants are carefully cultivated so as to enable the aphids to produce a maximum crop of galls. Quantities of the galls have been shipped to the United States and other countries.

Host specificity is illustrated among the cynipid gall flies. In general, about go percent of the galls are produced on species of oaks, 5 percent on species of roses, and 5 percent on different genera of the composites. Among the aphids and coccids, a species is usually associated with a distinct genus or even species of plant.

Certain insects cultivate specialized types of plants, especially fungi. This type of propagation attains its highest development among subterranean and mound-building termites, which also have highly developed caste systems. Their termitaria may be entirely underground or they may extend to the surface or rise many feet above it, in which case they are formed of thick, earthen walls hardened by the salivary excretions used in their construction. The termitaria may vary from a foot in height and diameter for some species to great mounds, pillars, or chimneys 15 to 30 feet high and almost as great in diameter. The fungus gardens are distributed throughout the central portions of the mound in a somewhat irregular manner. Termitaria of these types are constructed by rather small tropical or subtropical termites that reach their highest development in tropical Africa, South America, south India, and Australia. The termites are nocturnal foragers and prey on various types of vegetation, which is comminuted and mixed with excreta. This forms the food for rearing the fungi upon which the termites subsist.

Leaf-cutting ants of the tribe Attii also cultivate fungi in much the same way as termites but feed only on the fungus hyphae. The so-called ant gardens of the Amazonian ants in the genera Azteca and Camponotus are prepared and planted and the crop utilized for food.

Harvester ants play an important part in the accidental distribution of seeds by collecting, carrying, and storing them for food in their nests.

As INSECTS HAVE BEEN UPON this earth millions of years longer than human beings it is to be expected that they have acquired specializations and adaptations not wholly understood by us. The degree of development among insects is extremely variable and difficult to measure by human standards. There has been much speculation concerning the faculties of insects. It is well known that many species of ants, bees, and wasps, especially those that have communal tendencies, show a high degree of differentiation and efficiency in organization and labor. They may be likened to living machines motivated by some unexplainable power defined as instinct, if not powers of reasoning and intelligence. Yet insects display many remarkable traits not wholly understood by man.

The high degree of organization and caste systems of social insects ( termites, ants, bees, wasps) have been investigated by entomologists, but there is still a great deal to be learned about them. Much has been written about the highly specialized attainments of ants, in particular, concerning their ability to build nests; their social organizations, caste systems, and slave making; their means of communication; methods of collecting, storing, and growing food; their art of defense and selection and procedure in producing queens; and their maintenance and tolerance of an assemblage of nurses, guests, satellites, commensals, paupers, scavengers, kidnappers, murderers, and assassins somewhat after the pattern of our human society.

E. O. ESSIG is professor of entomology, entomologist in the experiment station, and former chairman of the division of entomology and parasitology in the College of Agriculture, University of California, Berkeley. He has been a member of that institution since 1914. Following graduation from Pomona College in 1909, he has devoted most of his life to agriculture and especially to entomology, on which subject he has written more than 1,000 papers and four books. His specialty in that field is the taxonomic and economic study of aphids, which insects have recently come into prominence in agriculture because they are among the worst insect vectors of plant virus diseases.