INSECTS 



HYMENOPTERA 



PHYTOPHAGA 



Saw-flies and Gall-flies 



The Phytophagous Hymenoptera include the saw-flies, and, for convenience, the whole of the 

 gall-flies, though some of the latter are parasites or even hyperparasites on the larvae of the gall-makers. 

 The former derive their name from the characteristic structures possessed by the female, by means 

 of which an incision is made in some plant structure leaf, twig, tender stem, bark, or solid wood 

 for the reception of the egg. These ' saws ' present great variety in their details, so that it is possible 

 to identify most of the British species by these appendages alone. The larvae of the saw-flies are 

 exclusively vegetarian in their diet, but vary greatly in appearance and habit. The majority are 

 leaf-feeders, and many of these closely resemble lepidopterous larvae. Some are leaf-miners, a few 

 are leaf-rollers, and some are gall-makers. Leptocerus luridiventris is broad and flat, and lives closely 

 attached to the leaf like some huge scale-insect. Eriocampoides /imacina, from its appearance and 

 manner of feeding, is almost invariably identified in the nursery and garden as a slug. The burrowers 

 in wood are very similar in outward appearance to the larvae of Coleoptera. 



The majority feed up during the summer and autumn and emerge in the spring. Many 

 appear during the summer, and Emphytus serotinus is taken regularly at Bishop's Wood, Truro, in 

 October. Some again are double-brooded, and some, like Athalia spinarum, may give rise to three 

 generations in the year. 



Saw-flies are for the most part sluggish and quiet, and though common, are rarely seen on the 

 wing. Their flight is weak and heavy, and in dull weather they will remain almost motionless on 

 leaves or flowers for hours at a stretch. Tenthredo, Allantus, and Cephus are common visitors to the 

 Umbelliferae and composites in the Public Gardens at Truro when the weather is bright and warm. 

 In the spring willow-catkins are in favour, and in the late summer Angelica becomes very attractive. 

 In 1901 one of the best hunting-grounds for saw-flies around Truro was an old rhubarb bed that 

 had been allowed to come into flower. A large triangular patch of hogweed near Malpas has been 

 a much-favoured spot for several years. 



One of the most remarkable facts connected with the saw-flies is the prevalence of partheno- 

 genesis. In nearly all species the males are very much less numerous than the females, and in many 

 no males have been discovered at all. Extensive observations and experiments on unfertilized eggs 

 obtained from females in confinement show that in most cases the parthenogenetic progeny is 

 entirely composed of females, in a few of males, and occasionally contains both sexes. Cameron is 

 of opinion that parthenogenesis involves constitutional weakness, fewer of the young from unfertilized 

 eggs reaching maturity than from those that are fertilized. Sharp says it appears most probable that 

 the parthenogenesis and the sex of the offspring produced by it are due to physiological conditions of 

 which we know little, and that the species in most cases continue in spite of parthenogenesis rather 

 than profit by it. Several species have, nevertheless, been shown to possess the power of reproducing 

 their like throughout a long series of generations without the intervention of the male, and as 

 Cameron acknowledges that there are over a hundred British saw-flies of which he has never seen 

 the males, there is very strong presumptive evidence that many species must owe their continued 

 existence to parthenogenetic reproduction. 



Though saw-flies, as a rule, are not much in evidence, there are several species that at times do 

 considerable injury in the larval stage to farm and garden crops. Of these the gooseberry caterpillar, 

 Pteronus ribesii, is probably the best known, as there are few gardens in the county it has not visited 

 at one time or other. When left unchecked it often completely denudes both gooseberry and 

 currant bushes of their foliage. Fortunately their skin is very susceptible to irritants and poisons 

 generally, so that a dusting of quicklime or of hellebore powder is an excellent remedy. It should 

 be followed, however, by the autumn removal of surface soil from under the bushes. 



In May, 1901, some pale yellow larvae with seven pairs of ventral legs were sent in from 

 Bodmin feeding inside young apples. Later on a number of the apples about the size of crabs, with 

 the inside partly consumed, fell from the trees, but the larvae themselves had vanished. An attempt 

 was made to breed out those originally sent, but this proved unsuccessful. They probably belonged 

 to the genus Hoplocampa, though no representative of the genus has been captured in that district. 



The foliage of the cherry, and occasionally of the plum and apple, suffer from the ravages of 

 the slugworm, Eriocampoides /imacina ; and in 1902 a number of thorns near Truro lost almost all 

 their leaves from such an attack. In 1897, an unusually dry year, some cherry trees at Liskeard 

 were completely stripped of their foliage in less than a week. In 1899 and 1900 pears and apples 

 were badly attacked about St. Columb and at Newquay, and a wall peach at Carworgie suffered 

 severely both years. In 1904 and 1905 the pest appeared in considerable numbers in the Truro- 

 Falmouth district. A liberal application of lime and soot well mixed together and washed off the 



'73 



