178 



KNOWLEDGE. 



[August 1, 1898. 



"INSECT MINERS." 



By Feed. Enock, f.l.s., f.b.s., etc. 



INSECT miners, though somewhat rare in the " Black 

 Country," are only too plentiful in the London dis- 

 trict. Being no respecters of persons, they invade 

 even Royal gardens, where we find whole families 

 of them working together in the most orderly, 

 systematic, and determined manner. I might truthfully 

 say that thousands of these insects are brought into 

 London every morning by ship, road, or rail. 



Let us take one of the favourite flowers of the Londoner 

 — the white Marguerite— a flower to be seen in every 

 street from Belgravia to Whitechapel. When the first 

 crop of flowers begins to fade, and the leaves are thus ex- 

 posed to view, those of us who have eyes for such things 

 can at once detect a peculiar appearance about the leaves. 

 Many of them are disfigured 

 with variously shaped yellow- 

 looking markings. Some peo- 

 ple imagine the plant to be 

 dying, and hasten its end by 

 consigning it to the dustbin. 

 Others wonder what is the 

 matter with their Marguerite, 

 and frequently set to work 

 and wash the plant — which is 

 a very good plan, as it invi- 

 gorates the growth, causing 

 new shoots to form as well as 

 buds, but it does not prevent 

 the miners going on with 

 their work, and even extend- 

 ing operations to the fresh 

 growth. 



Some few years ago I was 

 privileged to conduct some 

 lectures on economic ento- 

 mology for the Essex County 

 Council, and was much en- 

 Fia. 1. - Golden Marguerite couraged to find some of my 

 affected with " the Maggot," J [ audience bringing various spe- 

 cimens of insect and plant 

 life which had attracted their attention. The beautiful 

 yellow Marguerite was brought suffering from " the black 

 fly," which the grower could not get rid of from many of 

 his old stock plants ; and no sooner had he started a fresh 



stock for the 

 coming season 

 than this " fly " 

 made its appear- 

 ance, and, in many 

 instances, com- 

 pletely ruined the 

 plants for sale. 



Fig. 1 is from a 

 photograph of one 

 of these afflicted 

 yellowMarguerites. 

 The plant has 

 scarcely a sound 

 spray of leaves — 

 nearly all being 

 "eaten up" by the "fly," or rather maggot of this injurious 

 insect. I bred a large number of P. aj/inis (Fig. 2) from 

 the golden Marguerite. It is a minute, two- winged fly about 

 one-twelfth of an inch in length, slatey black, with black 

 bristles on head and thorax ; the head is pale yellowish 

 brown, legs dark, with yellowish tips and halteres. It has 



Fio. 



1. — The Marguerite Fly [Plii/totni/za 

 affinis). ( X 12 Diameters.) 



a decided objection to being watched, and some patience 

 is required to overcome the " now on the upper, now 

 on the lower " surface of the leaf; then, just when you 

 think you have it settled, it hops right away out of sight, 

 and you must wait for its return or seek another. 



Much patience an^ some gentleness of movement will 

 be required before you succeed in observing its method of 

 oviposition. After selecting a spot on the upper surface 

 it protrudes the rasped point of its telescopic ovipositor, 

 which it forces through the upper cuticle ; and then between 

 that and the lower (Fig. 4) it inserts an egg of an oval form. 

 Withdrawing its ovipositor it rapidly reverses its position, 

 and protruding its tongue proceeds to hermetically seal up 

 with saliva the aperture made (Fig. 5). Numbers of eggs 

 are so laid in each leaf, great care being taken to carefully 

 seal up each one. The next day minute blisters appear 

 over the egg, which hatches on or about the fourth or fifth 

 day into a tiny legless maggot, our first representative of a 

 " miner " (Fig. C). Nature has taught it that it must 

 work for its hving, and being provided with the necessary 

 muscular power it immediately commences to use its 

 excavating tools, which are in its mouth (centre of Fig. 6). 

 With these tools it obtains nutriment, and at the same 

 time levers its way between the cuticles, the "working" 

 gradually though slowly increasing in width and length. 

 In the yellow Marguerite it is straighter than is the case 

 in the broader leaves of the white Marguerite, where it not 

 unfrequently follows the serrated margin of the leaf for a 

 distance of an inch or more. At other times the insect 

 works a somewhat tor- 

 tuous course across the 

 leaf, and occasionally, 

 after making a narrow 

 mine, it seems to hit 

 upon an exceedingly 

 nice-flavoured piece of 

 leaf, around and about 

 which it lingers and 

 makes a well-formed 

 harbour (Fig. 7). The 

 course of the mining 

 larva is marked by the 

 minute pellets of frass 

 which are plainly visible 

 through the bleached 

 upper cuticle (Figs. 7 

 and 8). In less than a 

 fortnight the miner has 

 finished its excavation ; 

 its work, so far as yia. 3. -Leaves of Oolden Marguerite, 

 mmmg is concerned, is showing Larrs of rht/tomyza affinis. 



done, and its tools are (Natural size.) 



no longer required. At 



the head of the larva (left hand of Fig. 6) will be noticed 



Fig. 4. — Marguerite Fly ovipositing in Leaf. ( \ 12 Diameters.) 



two short prominences. These appear to be used as 



