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THE WILTSHIRE ARCHAEOLOGICAL AND NATURAL HISTORY MAGAZINE 



protruberances near the apex of the thorax known 

 as cenchri.The cenchri contact a rough area on the 

 underside of the forewings when the insect is at 

 rest, holding the wings in place. The Cephidae or 

 stem-sawflies are proportionately more slender 

 insects without cenchri but the abdomen is still 

 joined to the thorax without a narrow constriction. 

 All of these characters can be seen in Fig. 1 which 

 illustrates a pinned female Tenthredo thompsoni. 

 Some species of sawflies are as small as 4mm in 

 length whilst the largest can be up to 40mm. There 

 is much variation in the shape, proportions and 

 colour. Black, yellow, green and red in various 

 combinations are all quite common. Many adult 

 sawflies visit flowers where they feed on pollen, 

 nectar or parts of the flowers. Others are 

 carnivorous and catch insects including other 

 sawflies in their powerful jaws. Sawflies cannot sting 

 but the larger species are capable of inflicting a bite 

 when handled. 



The larvae of all the British sawflies feed on plant 

 material including horsetails, ferns, grasses, sedges, 

 herbaceous plants and many trees. Feeding mostly 

 takes place in the open on leaves but some groups 

 feed in other ways such as leaf mining, leaf rolling 

 or even by inducing gall formation. The wood- 

 feeding activities of the Siricidae have already been 

 mentioned and another group of species have larvae 

 which feed within the stems of plants. The larvae 

 of the species which feed in the open are seen most 

 commonly. Some of these feed singly and usually 

 have cryptic coloration whilst others feed 

 gregariously. This latter type are often brightly 

 coloured with striking markings and when disturbed 

 assume an alarm posture by lifting their tails to 

 form a rigid 's'-shape. There is some evidence that 

 some species may also employ chemical defence 

 substances. Individual species of sawflies are usually 

 very specific in their choice of host plant which is 

 an important aid to identification. The grey larvae 

 with black heads found on cultivated and wild 

 Solomon's Seal, for example, are always of the 

 species Phymatocera aterrima. 



The larvae of species feeding on leaves all have 

 three pairs of thoracic legs, like caterpillars of 

 Lepidoptera.They can be separated from that group 

 immediately, however, by the larger number of 

 abdominal pro-legs, usually six or seven pairs, 

 whereas lepidopteran caterpillars never have more 

 than five pairs. The larvae of sawflies which feed in 

 stems or in wood do not have well developed pro- 

 legs like the external feeders, but these larvae are 

 seldom seen. 



Fig. 2. Adult female Arge pagana ovipositing in a 

 stem of cultivated Rosa 



Fig. 2 shows a female Arge pagana ovipositing 

 in a stem of cultivated Rosa, and a group of the 

 gregarious larvae of this species is shown in Fig. 3. 

 The eggs of most species hatch within a few days 

 and the larvae then commence to feed. The newly- 

 hatched larvae of some species feed within the cavity 

 created by the saw of the female for several days 

 before eating their way to the external surface of 

 the leaf. There is considerable variation in the length 

 of time spent in the larval stage. Many species have 

 just a single generation in a year and others several 

 generations. Any larva alive at the end of the 

 summer spends the winter as a pre-pupa either in a 

 cocoon or in the litter beneath the food-plant. The 

 pre-pupa looks like a frozen waxy form of the larva 

 and this winter suspension of activity is known as 



Fig. 3. Group of larvae of Arge pagana 



