H. H. P. Severin, H. C. Severin, and W. Hartung.
The Stimuli which Cause the Eggs of the Leaf-ovipositing Tachinidae to Hatch.
Psyche 22(4):132-137, 1915.

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132 Psyche [August
REFERENCES,
Laboulbfine, A. (1872.)
Metamorphoses de la Puce du Chat (Pulex felis Bouch6)- Ann. Soc. Ent. France, Ser. V. II., 267-274. Kunckel. J. (1873.) Observations sur les Puces, en particulier sur les Larves des Puces du Chat et duLoir (Pulexfelis et Pules jasciatus). Ann. Soc. Ent. France., Ser. V. 111,129-142.
Taschenberg, 0. (1880.) Die Flohe. Halle, pp. 122. Packard, A. S. (1894.) On the Systematic Position of the Siphonaptera, with Notes on their Structure. Proc. Boston Soc. Nat. Hist., XXVI, 312-355. Mitzmain, Maurice B. General Observations on the Bionomics of the Rodent and Human Fleas. U. S. Public Health Bull., No. 38,1910. Russell, Harold. The Flea. Cambridge, 1913. Bacot, A. W. and Ridewood, W. G.
Observations on the Larvae of Fleas. Para- sitology, Vol. 7, No 2, June, 1914.
Infantile Paralysis in Massachusetts, 1907-1912. Monthly Public Health Bulletins, Mass, 1909-1915. THE STIMULI WHICH CAUSE THE EGGS OF THE LEAF- OVIPOSITING TACHINIDB TO HATCH.
Some difference of opinion exists as to the stimuli which cause the eggs of the leaf-ovipositing Tachinidae to hatch. The eggs of
the leaf-ovipositing tachina contain fully developed larvae at the time of deposition and are ready to hatch when swallowed by the host. Townsend (4, p. 109) suggests "that the digestive juices and conditions which the egg encounters in the alimentary canal of the caterpillar act upon the chitin and cause the shell to weaken so as to release the maggot." In direct opposition to this view, Swezy (3, p. 29) writes as follows: "The shell of the egg is so hard that it seems unlikely that it could,be sufficiently affected by the digestive juices of the caterpillar, quickly enough to allow the maggot to escape from the egg, and also have time enough to pass through the wall of the alimentary canal before it would be carried along and be expelled with the frass of the caterpillar." Swezey (2, p. 8) first ventured the opinion that the eggs "are so small as to escape being injured by the jaws of the caterpillar in biting off bits of leaf," but in a later paper (3, p. 30) he changed his view



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19151 Severin, Severin and Hartunq-Leaf-ovipositinq Tachinida 133 and believed that the mandibles of the host crack the egg-shell, allowing the maggot to escape. It appears to us that Ewezey's explanation of the method of hatching of the egg is faulty in some respects. If the jaws of the host were to exert too much pressure upon the egg-shell the maggot would be crushed. Again, by what mechanism would those eggs hatch which were swallowed, but did not first have the chorion cracked by the mandibles of the host? In order to obtain the eggs of the same species of tachinid (Chce- togocdia monticola Bigot) with which Swezey worked, a careful study of the egg-laying habits of this insect under natural condi- tions was made in the Hawaiian Islands. The fly usually deposits its eggs on different species of grasses and weeds. In observing the behavior of the parasitic fly, one is at first inclined to believe that the female searches for cutworms in the grass. The insect often crawls down into the thick matting of grass and disappears. Sooner or later it reappears, crawls up a blade of grass and stops a moment to deposit an exceedingly minute, black egg. As the tachinid crawls towards the tip of the blade of grass, the leaf bends, and it then grasps another blade of grass, crawls down, stops a moment to oviposit and again it may disappear in the thick mat- ting of the grass. Occasionally the creature may rest to go through cleaning movements -or to lap up some food. Often Chseto- gsedia takes short flights, only to resume egg deposition in a new locality. When disturbed, it flies away with a loud buzz, so quickly that the eye can hardly follow it. To obtain the eggs of this leaf-ovipositing tachinid, specimens of Chsetogsedia were captured in the field and placed in a glass jar provided with a cover. The bottom of the jar was covered with moist filter paper on which was placed a wet sponge. The cover was propped up with a toothpick to allow circulation of the air and thus prevent the accumulation of moisture on the sides of the jar. The food that was placed in the jar consisted of the pulp of ripe bananas and sugar. Under these conditions the flies de- posited hundreds and hundreds of eggs on the moist filter paper. The pulp of the banana seemed to be especially attractive as a medium for egg-laying, for the fruit was often blackened by hun- dreds of tiny eggs.
A number of experiments were performed to determine whether the eggs of the leaf-ovipositing tachinid would hatch in the juices



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134 Psyche [August
emitted from the mouths of a number of its hosts. The bodies of army worms (Heliophila unipuncta Haw.) were seized between a pair of broad, pronged forceps, whereupon the larvse ejected through the mouth a green liquid which was accumulated in a con- cave, center slide. One hundred eggs which had been deposited on moist filter paper by Chsetogsedia were emersed in this green fluid. The liquid containing the eggs was covered with a cover glass, care being exercised that no bubbles of air remained beneath the cover glass. In less than one minute some of the eggs began to hatch. Three hours later the larvse were counted and it was found that ninety-seven of the one hundred eggs had hatched. The green liquid was tested with litmus paper and gave a decided alkaline reaction. The eggs of Chc~togoedia monticola also hatched in the akaline juices exuded from the mouths of the following hosts: Vanessa cardui Linn. and Plusia clzalcites I3sp.l A similar experiment was performed with milkweed caterpillars (Anosia plexippus Linn.) from which the parasitic fly has not been bred, but the eggs hatched in the alkaline juices emitted from the mouth of the larvae.
The eggs of this tachinid were fed to the imported cabbage worm (Pontia rap@ Sch.) and the larvae of Heme cingulata Fab. ( =Sphinx convolvuli Linn.) from neither of which Chsetogsedia has been bred. A few days later, these caterpillars were dissected and the para- sitic larvae were found embedded within the fat tissue of the body wall.
The next problem which we attempted to solve, was to determine in what part of the alimentary tract of the host the eggs of Chse- togsedia hatch. A caterpillar of the milkweed or monarch butter- fly was fed a dozen eggs which had been placed upon a milkweed leaf. The caterpillar was vivisected immediately after the last egg was swallowed and three empty egg shells were found in the fore- intestine and nine in the mid-intestine. An army worm was viv- isected and eggs of the parasitic fly were introduced into the lumen of the fore-intestine and mid-intestine. Under a binocular micro- scope, maggots were observed emerging from these eggs. As there was a possibility that the digestive juices were regurgi- 1 Chcetogcedia monticola has been bred from fourteen different hosts in the Hawaiian Islands. "Kirkaldy (1, p. 49) reports a pupa of the butterfly, Vanessa cardui Linn. parasitized by this fly" and Swezey (1, p. 49) bred this tachinid from PZusia chalcihs Esp.



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19151 Severin, Severin and Hariung-Leaf-ovipositing Tachinida 1 35 tated from the mid-intestine into the fore-intestine in the cater- pillars used in the previous experiment, a host was selected which did not exude a liquid from the mouth when handled. Several mature, male walking-sticks (Diapheromera femorata Say) were fed bits of hazel-nut leaves upon which the eggs of a leaf-ovipositing tachinid (Phasmophaga antennalis Towns.) were glued. Each of two walking-sticks, which had swallowed twelve eggs of the parasitic fly, was injected with 95 per cent. alcohol three hours after feeding and was then dissected. All of the eggs were found in the fore-intestine at some distance in front of the oesophageal valve but not one of the eggs had hatched. Another specimen, which had swallowed seventeen eggs, was dissected five hours after feeding. In this case, nine eggs were found in the posterior region of the fore-intestine but none of these had hatched; eight eggs were taken out of the mid-intestine in the region of the glandular appen- dages and seven had hatched. Only one larva was found within the contents of the mid-intestine. Upon clearing and mounting the entire contents of the abdomen, it was found that six maggots had bored through the mid-intestine; four of these were embedded in the fat tissue of the body wall and two were found in the testes. The question naturally suggested itself, will the eggs of Chato- gcedia monticola hatch in an acid liquid regurgitated from the mouth of an insect? The liquid ejected from the mouth of a katydid gave an acid reaction, and eggs were emersed in this medium. A .
few minutes later one of the eggs hatched. The blood of army worms was extracted and gave an acid re- action.
From one hundred eggs emersed in this liquid, forty-five larvse emerged.
Of thirty-five eggs placed in the blood of the caterpillar of the milkweed butterfly only two hatched. It is evident than many eggs failed to hatch in the various acid media. In another experiment the eggs of Chsetogsedia were emersed in the acid juices pressed from blades of grass upon which the fly deposited its eggs, but not one of the eggs hatched. An attempt was made to hatch the eggs in a weak, alkaline so- lution of sodium hydroxide. After experimenting with a number of formulas it was found that 100 eggs eniersed in a .1 per cent. solution of sodium hydroxide, eighty-five hatched and of the same number of eggs emersed in a .05 per cent. solution of sodium hy- droxide seventy-two hatched. After the eggs had been emersed



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136 Psyche [August
in these alkaline solutions for twenty-four hours, some of the larvae, still enclosed within the vitelline membrane, protruded from the egg-shells. Sometimes the posterior part of the larva within the vitelline membrane protruded from the egg-shell, whereas in a normal hatching the anterior end pushes forth from the egg. Of the two hundred eggs emersed in the alkaline solutions, twenty- seven larvae emerged from the vitelline membrane within thirty- six hours, ninety-seven within forty-eight hours and thirty-three within sixty hours. In the experiment with the alkaline juices regurgitated from the digestive tract of the army worms, the eggs began to hatch in less than a minute, and ninety-seven of one hun- dred eggs hatched within three hours.
Eggs were now emersed in distilled water and results were ob- tained which were similar to those derived from the use of dilute solutions of sodium hydroxide. It appears, seemingly, that in- creased turgidity causes the larvae to emerge when the eggs are emersed in water or dilute alkaline solutions for thirty-six hours or longer.
Is there a physiological mechanism connected with the hatching of Chcetogcedia monticola? A little pressure exerted upon the eggs of this tachinid causes the maggot enclosed by the vitelline mem- brane to issue from the chorion apparently uninjured. If the larvae within the vitelline membrane is emersed in the liquid exuded .
from the mouths of army worms, the process of hatching can readily be observed under a microscope. As soon as the maggot enclosed within the vitelline membrane is emersed in the liquid exuded from the mouths of the army worms, bodily contractions can be observed, in a minute or less the contractions become more energetic and one can readily see the segments move from the pos- terior to the anterior end of the larva resembling a peristalic movement. Next the larva turns the anterior end towards the flattened or attached side of the egg so that the jaws are near the posterior end of the abdomen. The maggot then attempts to cut the vitelline membrane with the sickle-shaped part of its jaws. Often the distal end of the jaws appear to slip on the inner surface of the membrane, but once the end of the jaws is pushed through the vitelline membrane, a single sweep of the head, and the jaws cut a longitudinal incision, through which the maggot makes its way out. In pushing out of the egg-shell, peristalic movements of the body segments are again evident.




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