210 NATURE-STUDY REVIEW [12:5— May, 1916 



tecting the young not only from parasites but also from extremes 

 of temperature and moisture. 



Many gall insects secrete a sweet, sticky substance which is 

 particularly attractive to ants. The latter act as sentinels 

 to guard the gall insects from parasites. 



The general distastefulness of galls probably also protects them 

 from many animals that would otherwise be their enemies. This 

 is due either to the large amount of tannin in the tissues of the 

 gall or to the emission by the larva of a disagreeable odor. 



Agents Concerned in the Production of Galls. 



The extent to which plants are infested with galls may be 

 realized when a list is made of those found upon a single species 

 or groups of species. The oaks, for example, are visited by a 

 large number of different species of gall insects. Thompson in 

 his recent paper on American insect galls lists several . hundred 

 on this group of trees. They occur on the roots; attached to 

 twigs and branches ; or within the twigs and branches and on the 

 leaf buds, leaves, aments, and fruit. 



The producers of galls belong to five different orders of insects 

 and to several families under each order, as follows: 



I. Order Hemiptera — True bugs. 



i. Family Aphididas — Plant lice. (Figure 8). 



2. Family Psyllidae — Jumping plant lice (Figure 5). 



3. Family Coccidae — Scale insects. 



II. Order Diptera — True flies. 



1. Family Cecidomyidae — Gall gnats (Figure 1). 



2. Family Trypetidas — Trypetid gall-flies (Figure 9). 



3. Family Mycetophilidae — Fungus gnats (Figure 7). 



III. Order Coleoptera — Beetles. 



1. Family Buprestidae — Metallic wood borers. 



IV. Order Lepidoptera- — Butterflies and moths. 

 1. Family Gelechidae — Gall moths. 



V. Hymenoptera — Ants, bees, wasps, etc. 



1. Family Cynipidae — Hymenopterous gall flies (Figure 4). 



2. Family Tenthredinidae- — Saw flies (Figure 10). 



Besides the groups of insects listed above, certain of the mites 

 (Class Arachnida) produce galls (Figure 6) and eelworms (An- 



