May 1, 1897.] 



KNOWLEDGE. 



107 



Fig. 5.— Parasite of Willow Saw-Fly. 



immediately sucks dry. This first taste of food whets its 

 appetite, and it hurries out of its eggshell, to walk abroad 

 among the horde of AphiiLv, among which it soon makes 

 a clearing, and quickly increases in size, until in a few 



days it becomes a grub 

 of about three-eighths 

 of an inch long, having 

 an attenuated body, 

 a small hook-shaped 

 mouth, a pair of anal 

 claspers, the tail end 

 being much thicker 

 than the head. Its 

 favourite habit is to 

 affix itself by the 

 claspers to a midrib, 

 or any projection on 

 the under side of a 

 leaf. Here it waves 

 its body to and fro, 

 pitching upon some 

 aphis, which it imme- 

 diately seizes and hoists up into mid air, where it sucks 

 out every minute drop of fluid, until only the whitened 

 skin remains. This is flipped away. The larva immediately 

 takes up another aphis and sucks it dry, flings it 

 away, and again repeats the process, until its rapacious 

 appetite is appeased. A good healthy larva can easily 

 destroy from one himdred to oin- hundred and ticiuti/ gifi n 

 fies in one hour '. So that, during its life of ten to thirteen 

 days, some thousands of AjJiidip are destroyed. When full 

 fed it retires to a twig, dead leaf, or out-of-the-way spot, 

 on which it firmly fixes itself by the anal claspers ; the 

 attenuated body shortens, and widens tremendously at the 

 head imtil it becomes quite pear-shaped (Fig. 1) ; the 

 skin hardens, forming the protective covering to the 

 maturing fly, whose markings, as they reach perfection, 

 can be easily seen through the semi-transparent skin. A 

 transverse suture appears a short distance from the thick 

 end, denoting the near appearance of the fly, which, in 

 the course of a few days, splits this lid vertically and 

 horizontally, and by muscular effort quietly rises from 

 between these two small lids, which swing back on the 

 most delightfully contrived hinges. The large compound 

 eyes of the wasp fly occupy almost the entire head ; then 

 legs are gradually drawn out, and as soon as the first and 

 second pair are free, the fly takes hold of the stem or leaf 

 and lifts itself right out of the pupa case, which is left 

 adhering to the stem, while the fly walks round to the 

 under side, where it hangs by its claws until the fluid has 

 permeated the wings, when it gently takes its first flight, 

 hovering quietly over the flowers in search of its mate. 

 The nuptials over, if a female the business of its life 

 commences, and it quickly distributes its eggs over the 

 aphis-infested plants, and, its mission and purpose fulfilled, 

 it dies. 



One of the most interesting inhabitants of a London 

 " back " garden provided with willow trees is the willow- 

 gall saw-fly. Everyone without exception must have 

 noticed the willow leaves in June bespangled all over, 

 front and back, with these pretty, bright, crimson- coloured 

 little galls — but very few appear to care to inquire into 

 their origin. Some of our scientific men are in doubt 

 as to the insect which is the cause of them — it is astonish- 

 ing with what tenacity such cling to their books, instead 

 of going direct to nature. Of course, a three-volume 

 " work " can be read in a few days, but year after year 

 may roll by before the secrets of the life-history of a 

 common insect are observed and confirmed ; but, as a 



correspondent wrote to me the other day, " the study of 

 the life-histories of insects is so enthralling that one is 

 astonished that people have time for reading so much that 

 is unprofitable." 



Now, let us follow out the life-history of this willow 

 gall, which makes its appearance on almost every willow in 

 the country, as well as in the heart of London. It is the 

 wiUow saw-fly [Nematus (jallicola) which is the cause of it. 

 At the present time (April and May) the females of this 

 fly are engaged in laying the foundation of the galls. 

 In Fig. 2 the saw-fly is seen resting on one of the 

 young leaves, on which is to be seen a number of minute 

 slits ; these, as well as those on the half-expanded leaf 

 below, denote the small slits made by the ovipositor, which 

 is formed exactly like a most perfect tenon saw, having 

 exquisitely formed compound teeth. Each kind of saw-fly 

 has totally different teeth, which form admirable characters 

 for classification. For the number and names of saw-flies 

 see Mr. Cameron's " British Tenthredinidie " (Eay Society). 

 This wonderful organ not only serves the purpose of a saw, 

 but also that of a perfect tubular ovipositor, the two blades 

 and tenons being so wonderfully " rabbetted " together 

 that from being almost a flat saw it can be so expanded as 

 to permit of an egg passing down and being conducted to 

 its nidus between the upper and lower cuticle. Fig. 3 

 shows this organ dissected to display the various parts, all 

 of which, when out of use, are carried in the sheaths above. 

 Immediately the egg is laid the fly withdraws her ovipositor, 

 turns round, and applies her mouth to the minute slit, 

 which she seals with saliva. 



I have frequently counted between twenty and thirty 

 galls on a single willow leaf. A few hours after the eggs 

 have been laid, a change in the colour of the leaf takes 

 place, and a rapid growth resembUng a small pimple 

 commences, which, in a few days, increases to a gall of 

 about three-sixteenths to a quarter of an inch in diameter, 

 swelling out from both surfaces of the leaf — that on the 

 upper gradually assuming a most brilliant crimson colour. 

 The lower part is not so decided in colour. 



Some authorities state that the egg hatches almost 

 immediately — before the gall is formed — but such has not 

 been my own experience, as I have repeatedly cut open 

 galls of three days' growth and found the egg in the centre, 

 and not until the fourth day have I found the young larva. 

 The first food taken by the occupant is from the cellular 

 structure of the gall, from the centre of which to the imder 

 side a small tunnel is eaten out. This serves the purpose of 



Fia. (i. — Parasite of WiUow Saw-Fly piercing Grail. 



a channel and ventilator — all waste material falling out at 

 the aperture, which is increased in size when necessary. 

 The growth of the larva is somewhat slow, and though the 



