210 INSECT TRANSFORMATION 



in summer and early autumn may often be seen small spherical 

 bodies like succulent fruits, greenish yellow in colour, with 

 red markings ; these are galls of Pontania salicis, and each 

 contains a chamber within which the larva finds protection 

 and nourishment. In correspondence with its concealed 

 dwelling-place this larva is pale in colour ; it feeds on the modi- 

 fied plant tissue of which the gall is built, and when fully fed, 

 it descends to the soil for pupation. Next spring the flies 

 black and yellow insects with clear wings emerge from the 

 buried cocoons and lay their eggs in incisions, cut with their 

 serrate ovipositors below the leaves. The plant-tissues are 

 irritated by this action, or by the pressure of the egg, so that 

 they respond by developing the characteristic and regular 

 gall, which comes to surround the eggs ; thus the larva when 

 hatched finds itself provided with abundant food and effective 

 shelter. Other saw-flies of the same genus (Pontania) have 

 larvae that live in galls on the leaves and shoots of willows. 



The gall-forming habit is a highly interesting subject of 

 study in connexion with the life-relations of insect-larvae. 1 

 A gall may be regarded as a pathological growth, a diseased 

 excrescence due to the presence of the egg or the larva in the 

 plant-tissues ; but, by producing a gall, the plant provides 

 food for the insect at less expense to itself than by letting the 

 insect devour indiscriminately its leaves and shoots. Only 

 a comparatively few saw-fly caterpillars are gall-feeders, but 

 the great majority of larvae of another hymenopterous family, 

 the Cynipidae or Gall-flies, distinctively live in this manner ; 

 and the oak is the tree that harbours the greatest variety of 

 them. The familiar " oak-apples ", the fruit-like " cherry- 

 galls " below the leaves, the beautiful " spangles " or button- 

 galls, the curious cottony growths on the shoots are all due to 

 the presence of various cynipid larvae. Although the female 

 gall-fly has a long piercing ovipositor by means of which she 

 inserts her eggs into the tissues, it is apparently neither the 

 irritation due to the act of egg-laying nor the presence of the 

 egg, but a secretion of the larva after hatching which serves 

 as the stimulus leading to the responsive gall-forming growth 

 on the part of the plant. It is remarkable that the larvae of 



1 C. Houard : Les Zoocecidies des Plantes d' Europe et du Bassin de la 

 Mediterranee ". Paris, 1908-13. E. T. Connold : " Plant-galls of Great 

 Britain ". London, 1909, 



