Thursday, 28 April 2016

Sawflies

Sawflies (Symphyta) are a very understudied group of insects. They belong to the Hymenoptera, which includes Wasps, Bees and Ants, but the female has a saw instead of a sting, hence the common name. The saw is used to cut slots in leaves, and she then lays her eggs into the slots as an anchor to hold them in place, often laying many eggs on a single leaf. The larvae look very much like the caterpillars of moths and butterflies, but they have more prolegs and are generally more tapered, with wider 'shoulders' and narrower rear ends. They also have very simple ocelli on the larger eyes, only a single dot rather than multiple dots as found on moth larvae.

Larva of Arge gracilicornis
The eggs are laid on particular plants, sometimes a single species of plant, but often within a closely-related family of plants. Some species are leaf miners and some others make galls, although the majority of larvae are free-ranging.

I'm not quite sure why Sawflies are so studiously ignored by many entomologists. True, there are many species and many are very similar to each other, often requiring a specimen and a microscope to identify accurately, but other groups are like this, so I'm not quite sure why they are seen as 'difficult'. I suppose it must be a lack of literature: there is very little out there: the main key (Benson in 3 volumes) is now over 60 years old, but I have found the key to be no more difficult than other groups such as beetles or flies. One difficulty is the larvae: they go through a series of moults (instars) as they grow, and can vary their colour pattern quite substantially when they do so. So in order to identify the larvae we need to know all the variations for each species. Cameron's four-volume monograph, written in the late 19th century, includes coloured drawings of some larvae. A further difficulty is that we don't yet have a full knowledge of which larvae turn into which adults. With moths and butterflies, the match is more or less complete, since a great deal of work has been done with them and they are relatively easy to breed through in captivity, but with sawflies the life cycle can sometimes be a bit more complex, and many attempts to raise them in controlled conditions have failed. So we have a situation where some larvae are as yet unidentified, and the larvae of some of the adults are unknown. Clearly, much work needs to be done here.

Adult sawflies can be difficult to distinguish from other groups, but they always have a thick waist like bees, rather than a narrow one like wasps. The thorax is often strongly sculptured.

Tenthredopsis nassata
 
Tenthredo livida, male
Larvae often adopt a tail-up stance when disturbed. The movement is quite sudden, and I have seen parasitoids being propelled through the air if they approach too closely. This is the larva of Nematus pavidus, which is a common Willow feeder:

Larva of Nematus pavidus

The differences between species are often microscopic, and we need to examine a specimen under magnification. With some species we need to see further details, such as the shape of the teeth on the saw. This is the (2 mm,) saw of a Tenthredo:

Saw of a female Tenthredo sp. Sawfly

A few years ago I studied a series of colonies of Nematus pavidus on my local Willow and it is documented here.

Sawflies certainly need attention, and I intend to focus on them this year.

2 comments:

Gill said...

Fascinating. What are the saw teeth made of - hard chitin like the exoskeleton?
Why are they so much harder than butterflies to breed through - do they change diet part way through or something?

stuart dunlop said...

Gill: yes, the saws are hard. Some saws are used to cut grooves in twigs, so they are thicker and more robust. The larvae of species with a summer generation can be bred through quite easily, since they progress rapidly from larva to pupa to adult, but many species overwinter as pre-pupal larvae and they are much more difficult to breed through successfully.