R. C. Harris.
Occurrence, Life-cycle, and Maintenance under Artificial Conditions, of Miastor.
Psyche 30(3-4):95-101, 1923.

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PSYCHE
VOL. XXX. JUNE-AUGUST 1923 Nos. 3-4
OCCURRENCE, LIFE-CYCLE, AND MAINTENANCE, UNDER ARTIFICIAL CONDITIONS,
OF MIASTOR.
From the Laboratoire d'4volution des &res organises, Paris, Prof. Caullery, Director.
The genus Miastor is extremely interesting for several rea- sons.
It was in this genus that paedogenesis was discovered by Nicholas Wagner in 1861.
Though at first Wagner mis-
interpreted the phenomenon which he observed, and though his observations were doubted; subsequent investigation has dem- onstrated the fact of paedogenesis as well as the occurrence of polymorphism in this genus. But investigations upon Miastor have been limited, due to some extent at least, to the lack of a suitable method of maintaining the larvae of this genus in the lab- oratory. Now that a method, which will be discussed later, has been found, it seems to the writer desirable to make a short re- sume of the occurrence of Miastor, its life cycle, and polymor- phism among the larvae of the genus.
Occurrence of Miastor.
Miastor larvae were first observed underneath the bark of decaying trees. I have found them in France under the bark of decaying oak, chestnut, and birch logs, and in edible mush- rooms.
Other observers have found Miastor larvae in many kinds of decomposing wood and in fermenting beet pulp. It may be safely concluded that their occurrence is fairly widespread in decaying logs and that they are sometimes present in other vege-



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96 Psyche [~une-~u~ust
table matter.
Their habitat is not limited to Europe. In Amer- ica Felt1 found Miastor larvae in 1910 under the partially decayed inner bark of chestnut rails.
In such environments Miastor larvae usually occur in charac- teristic, compact, white colonial masses, a colony often con- taining a hundred or more individuals in close contact with each other. Among Miastor larvse occurring in mushrooms I have not observed colonial .arrangement,. Here yellow or yellowish white larvae are found singly although a single plant may con- tain many larvae directly beneath the superficial cell-layers of the stalk or between the gills of the umbrella. The Life-Cycle of Miastor.
Psedogenetic reproduction maintains in this genus during the autumn, winter, and spring, until the early summer, at which time pupae occur, producing, after meta~morphosis, male and female imagines. (The occurrence of pupae, however, is not limited to the early summer. I have found them in nature in October, and in the same material pupse continued to arise in the laboratory during the early winter. Though these were kept in conditions as natural as possible, at room-temperature, none of the pupae has as yet given adults, though the usual period required for this metamorphosis in Miastor is about five days.
Some of the pupae have been destroyed by mould. It
would seem that others, apparently in good condition, are in diapose.) The copulation of the adult flies results in the fer- tilization of the eggs which develop outside the mother into typical paedogenetic larvae, and the cycle is complete. Psedogenesis in Miastor is realized as follows: The ovaries of a typical paedogenetic larva produce eggs varying in number in different individuals, types and species. The development of larvae from the eggs occurs within the body-cavity of the mother. The embryos continue to develop at the expense of the fat- bodies, muscles and surrounding tissues of the mother larva until IFelt, E. P. Miastor americana, an account of , 1911 vol. 147, pp. 82ff.
Pedogenesis. Bull. New York State




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19231 Occurrence, Life-Cycle, and Maintenance of Miastor 97 a suitable growth is attained. The embryos seem to be suitably developed, and are usually active for several days previous to their escape. At the time of escape, the larva, while not full grown, is well formed, active, and capable of continuing its existence. The time required for a typical psedogenetic gen- eration is about two weeks, though the period varies with in- divuals and environmental conditions. White paedogenetic larvae produce four to ten or more embryos. Polymorphism in Miastor Larvce.
In all species of Miastor larvae, which I have observed, there occur three distinct larval forms.
Two are psedogenetic: viz.
white and yellow paedogenetic larvae, while the third is incapable of psedogenetic reproduction; it is the pupa-larva. (1) Typical white pcedoqenetic larva.
The body of the larva contains fourteen segments. In the first or head-segment are the mouth opening and two antennae. The mouth parts are arranged for sucking. The second and third segments contain the optic ganglia, and the third segment two eyes. In typical psedogenetic larvae the eyes usually touch each other at their convex surfaces. The brain is in the fourth and fifth segments. Dorsal and partially posterior to it is the brain fat body. The salivary glands are in the fifth and sixth segments on either side of the oesophagus, their ducts opening in the mouth cavity. The oesophagus extends posteriorly from the mouth and opens into the intestine in the fifth segment. In this region the intestine enlarges and folds, forming two blind appendices, which are in turn divided into two parts each. The digestive tube continues to enlarge slightly until it reaches the region of the tenth segment where it narrows into the rectum which is not functional in later stages. The intestine contains a peritropic membrane for inclosing undigested food. Four tubes of Malpighi extend anteriorly from the region of the twelfth to the tenth segment. The anus is in the fourteenth segment. Here are also two symmetrical anal flaps which are easily visible when protruded. The larva contains well developed fat-bodies,



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98 Psyche [June-August
filling much of the body-cavity from the sixth to the fourteenth segments. The two ovaries occur in the region of the tenth segments, and are closely connected anteriorly with fat-bodies. There are small chitinous points, arranged in four or more rows, extending around the larva between ea,ch segment from 2 to 14. Six chitinous hooks terminate the larva in the last seg- ment.
There are two tracheal trunks on each side of the body, one laterally ventral, one dorsal. These have numerous cross branches, viz., one large branch connecting the two dorsal trunks in each segment from 6 to 13. From each of these branches two other minute branches extend posteriorly in each segment. Numerous branches and sub branches proceed from the ventral trunks. he dorsal and ventral trunks are connected in each segment from 6 to 13, another branch opening to the exterior on each side thus forming eight pairs of stigmata. There being no pro- truding spiracles it would seem difficult for the larvae to remain long submerged. However, larvae remain alive for long periods of time, a fortnight or more, when completely submerged in water under a cover-glass.
A series of large ganglia occur ventral to the digestive tube- Locomotion results from stretching each segment and then contracting it, the process extending anteriorly. The rows of
chitinous points prevent the larvae from slipping. The size of larvae varies with species, individuals, cultures and age. New born paedogenetic larvae of Miastor metraloas, average 1.35 x 0.16 mm, while paedogenetic mother-larvae at- tain a length of from 3 to 4 mm.
2) Yellow pcedoqenetic larvae.
Yellow forms (wanderers) are similar in general structure to white psedogenetic forms. They are extremely active. The body is usually slimmer, and the fat bodies less heavily developed than in white paedogenetic larvae. The yellow pigment seems to occur for the most part in the fat-bodies. The eyes are notice- ably larger than in other Miastor larval forms, and approach closely along their surfaces.




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19231 Occurrence, Life-Cycle, and Maintenance of Miastor 99 Yellow larvae usually produce but a single embryo in the species (Miastor metraloas?) which I found in decaying wood. Yellow larvae (undescribed) occuring in mushrooms, however, produce a large number of embryos. I have frequently observed twenty embryos in a mother-larva.
Neither the role of, nor the incentive factor producing, yellow forms is clear.
In active yellow larvse I have observed a jumping locomotion. This seems to occur as a result of the larva bending its body until the anal chitinous hooks catch against the chitinous points of the second or third segment. Extension of the segments then tightens the tension which finally breaks with a snap re- sulting in a jump. The yellow forms are extremely active, and may often be seen lifting their heads and much of their bodies straight into the air.
3) Pupa-larva.
Larvae which will metamorphose into pupse are immedi- ately distinguishable from other forms by means of three char- acteristic differences, though in general their structure is similar to that of white and yellow psedogenetic forms. a) In newly born living pupa-larvae the imaginal discs are visible. These occur laterally in the third, fourth and fifth seg- ments. They are not present in paedogenetic forms. b) The spathula sternalis, a structure typical of Cecidomyid larvae, occurs ventrally in the third segment of the pupa-larvae of Miastor. It is not visible in newly born living pupa-larvae, but after four or five days becomes clearly visible, due to a yellow- ish coloration which later changes to orange and dark brown- The shape of this structure differs with various species. Its. function is not understood.
c) The eyes of pupa-larvae do not touch as in paedogenetic: forms but are usually clearly separated. The fat-bodies of pupa-larvae are extremely well developed* The larvae are active up to the time of pupation, unlike paedogene- tic larvae which of necessity become inactive with the progressive destruction of their muscles, due to the development of the embryos.




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Psyche [June-~u~ust
In Miastor as in all
cyclorrhaphous diptera, the pupa is
formed within the last larval skin. After the pupa-larva comes to rest the color of its skin changes from white to yellow, later to orange red and dark brown.
Under optimum conditions a period of about five days is required for the metamorphosis of pupae into imagines. The causes which lead to the occurrence of the various types of larvae within a single species of Miastor are not known. I
have observed all the various types occurring side by side in nature and in the laboratory.
Springer's2 belief that the yellow
forms and pupa-larvae are produced by the action of light upon typical psedogenetic mothers does not seem tenable, since I have observed all types occurring in the laboratory in cultures reared in the dark as well as in those reared in the light. Method of Culture in the Laboratory.
In order to carry on investigations concerning the factors regulating the life-cycle of Miastor and those causing polymor- phism among the larvae of a single species, it seemed immediately desirable to find a suitable artificial medium in which cultures might be reared in the laboratory under controlled conditions. With this in view I attempted to rear Miastor larvae on various media, finally meeting with success on a mushroom bouillon- agar innoculated with yeast.
The mushroom bouillon is made by taking equal parts by weight of mushrooms and water. The mushrooms are boiled in the water for 20-30 minutes. At the end of this time the mush- rooms are withdrawn from the bouillon to which is added suffi- cient agar to make 4 per cent of the bouillon weight. The whole is heated until the agar is dissolved, when it is poured into small mouthed stock bottles stoppered with cotton, and sterilized; in this way the culture medium may be kept indefinitely. When it is needed for use, the medium is liquified in a hot water-bath and while still hot poured into culture dishes, and allowed to ZSpringer, Fritz. Polymorphismus bei den Larven von Miastw metraloas. 2001. Jahrb Abth. f. Anst, vol. 40 p. 57 (1917).




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19-23] Occurrence, Life-Cycle, and Maintenance of Miastor 101 harden. For mass cultures of psedogenetic larvae I have found petri dishes very suitable for culture containers. Since they allow but little loss by evaporation, the water of condensation falling back upon the culture medium, the medium remains moist,, suitable for the reproduction of the yeast and the growth of the larvae for some time. Larvse may be more easily removed for examination from cultures reared in petri dishes than from those maintained in deep containers. Very small petri dishes may be used for individual cultures.
After the agar has hardened, a little powdered yeast is sprinkled over its surface, to which the larvse are transferred. Cultures are then placed in the incubator at 20-22 degrees Centrigrade.
I have employed this culture medium for Miastor larvae of various species, and have observed in colonies reared in this way all the larval forms occurring under natural conditions. It seems then that a method has been found for rearing cultures of Miastor larvse in the laboratory under conditions which may be easily regulated. This method should be suit- able for maintaining cultures of a psedogenetic insect in the lab- oratory for class work as well as for carrying on investigations concerning the factors influencing the. life-cycle and the poly- morphism of the larvae of Miastor.




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