Leopoldo B. Uichanco.
Reproduction in the Aphididae with a consideration of the modifying influence of environmental factors.
Psyche 28(4):95-109, 1921.

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PSYCHE
VOL. XXVIU AUGUST. 1921 No. 4
REPR.ODUCTION IN THE APHIDIDB WITH A CONSID- ERATION OF 'THE MODIFYING INFLUENCE OF
ENVIRONMENTAL FACTORS.1
BY LEOPOLDO B. UIOHANCO
College of Agriculture, University of the Philippines, Los Bafios, P. I.
Literature on aphids abounds in references to the existence of an "asexual" and a "sexual" reproduction. As a matter of fact,
however, they reproduce sexually only, the term "sexual reproduc- tion" being generally accepted by zoologists as meaning reproduc- tion by means of special reproductive cells, and "asexual reproduc- tion" that method which involves a direct division, or budding, of an animal without the intercession of specific germ cells. Reproduction in aphids may be subdivided into two categories: 1. Amphigony. This involves the union of reproductive cells of both sexes, the female gamete, or egg, necessitating fertilization by the male gamete, or spermatozoon, as a prerequisite to development. 2. Parthenogenesis. In this mode of reproduction the male ga- metes are dispensed with, the egg developing without having been previously fertilized. Hertwig and Kingsley (1912, p. 130) char- acterize parthenogenesis as "a sexual reproduction in which a degen- eration of fertilization has taken place." In addition to the two foregoing methods of reproduction, occasional cases of pcedogenesis (e. g., in Aphis avenue Fabricius, as reported by Ewing, 1916, and. in Toxoptera graminum Rondani, Webster and Phillips, 1912) have been reported. Padogenesis is parthenogenesis occurring in the preadult stages of animals.
Amphigonous reproduction is considered as the more primitive method in insects, parthenogenesis being the result of a later specialization. The latter method of reproduction has become of normal occurrence in aphids and has practically supplanted the former, amphigony taking place only under the influence of ad- verse conditions in the environment. We have thus in aphids a, Contribution from the Entomological Laboratories of the Bussey Insti- tution, Harvard University. No. HI.




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96 Psyche [June
very remarkable instance of a highly specialized group of sexually reproducing animals in which fertilization of the female reproduc- tive cells as a prelude to development has been reduced to an apparently unnecessary and unessential physiological process. In the amphigonous generation both sexes are, of course, repre- sented. The presence of the male is the only characteristic of the amphigonous generation.
The main external characteristics of an amphigonous female are the general absence of wings (Baker, 1920)) and the presence of an ovipositor in certain species (Buckton, 1882, p. 119). Davis (1908) also noted the presence of ^sensoria^ on the hind tibiae as a secondary sexual character of the amphigonous female, "at least in the subfamilies Pemphiginse, Schizoneurinse, Lachininse and A~hidinse.)'~ Internally, the most conspicuous features are the presence of a spermatheca and a pair of collaterial glands. The acessory glands, as well as the vagina, of which they are an evagi- nation, and also the oviducts, are easily recognizable on account of their relatively thick walls. The large amphigonous eggs and their nurse cells are also very characteristic and are in evidence in the ovaries early in the embryonic stage of the mother. Cleavage and the formation of the blastoderm do not begin until after the eggs are fertilized and deposited. The amphigonous female is oviparous. The eggs are covered with a vitelline membrane and chor ion.
.Among the aphids of temperate countries, aphidologists distin- guish between (1) "stem mothers," which are the parthenogenetic individuals hatching from the overwintering amphigonous eggs, and (2) later parthenogenetic generations. The former are typi- cally apterous; the latter, either apterous or alate. In the fall 2 The typical genus of Schizoneurinae, which is Schizoneura Hartig, 1837, is considered by Baker (1920) as a synonym of Eriosoma Leach, 1818. He grouped the latter under the tribe Eriosomatini of the subfamily Eriosoma- tinse. The subfamily Pemphiginas has been reduced by the same author to tribe Pemphigini under the subfamily Eriosomatinae; and Lachninse, to Lachnini under Aphidinse.




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Uichanco-Reproduction in the Ap71ididos
there is also a third form, the generation of ^sexupara," that is, parthenogenetic individuals which give rise to amphigonous forms. These are, as a rule, alate (Patch, 1920). In all specimens of
parthenogenetic aphids which I have dissected neither spermatheca nor colleterial glands were found. The absence of these structures was apparently first reported by von Siebold (1839) and subse- quently confirmed by other authors. In contrast with the amphi- gonous forms, the walls of the ovary are uniformly thin and mem- braneous, except at the vagina, where they are somewhat thicker. Parthenogenetic aphids are viviparous, the entire incubation pe- riod being passed within the abdomen of the mother. This is a very unique characteristic, in view of the fact that a similar case -does not occur in closely related families. The Phylloxeridse, which
is the only other family with the Aphididse in the superfamily Aphidoidea, also have parthenogenetic generations, but they are always oviparous. In certain other families of the order Homop- tera, like the Aleyrodidse, parthenogenesis and viviparity are not correlated.
In parthenogenetic aphids, the development of the eggs proceeds in the ovaries long before deposition, eggs in the blastoderm stage having been observed within the abdomen of parthenogenetic em- bryos. There is no formation of a vitelline membrane and chorion. As has been suggested elsewhere in the present paper, aphids seem to have the tendency indefinitely to reproduce parthenogenet- ically under favorable environmental conditions. As experimental
evidence favoring this view may be cited Ewing's (1916) work on Aphis avence Fabricius, in which he found that he could maintain continuous and uninterrupted parthenogenetic reproduction through as many as eighty-seven generations in the material he experi- mented with on the Pacific Coast of the United States. I11 that region, oviparous forms in any aphid species had not been known, except in very few cases. His experiments, unfortunately, had to "b brought to a close through the dying of all the individuals in his eighty-seventh generation from excessive heat; otherwise, he would probably have been able to observe parthogenetic reproduc-



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98 Psyche [June
tion for a much longer period, if not indefinitely. Earlier investi- gators, like Bonnet (1745) and Kyber (MIS), had previously ob- served the maintainence of continuous parthenogenetic reprodue- tion in various species of aphids for long periods of time. Males and oviparous females are known to be produced only under the prolonged influence of extremes of temperature, such as during the winter in temperate climates, and, as certain inves- tigators claim, in cases of scarcity of food. Observations on this modifying action of climate and food, according to Buckton (1882, p. 109) were reported by Bonnet in 1745, and later confirmed by De Geer in 1773.
From our somewhat fragmentary knowledge of tropical aphids, we may tentatively infer, in the absence of more definite evidence to the contrary, that representatives of this family reproduce exclusively by parthenogenesis in warm countries. The following table gives a partial list of the localities in which a continuous parthenogenetic reproduction throughout the year has been ob- served.
It will be noted that the various localities included in
the list are characterized either by the total absence of winter or by a relatively mild climate.
Holland
India
LOCALITY SPECIES AUTHORITY
France (Orleans) Aphis rumicis L. Gaurnont (1913) Germany (Bremen ?) Hyalopterus trirhodus Walk. Borner (1914)
Rhophalosiphum lac-
tucce Kalt. ditto
Aphis gossypii Glov.
van der Goot (1915)
Aphis Izederce Kalt. ditto
Aphis abietina Walk. ditto
Aphis rumicis L. ditto
Eriosoma lanigera Hausm. ditto
Macrosiphum granarium
Kirby ditto
Myzus persicce Sulz. ditto
-4phis brassicce L.
Maxwell-Lef roy ( 1907)
Aphis cardui L. var. ditto
Aphis gossypii Glov. ditto




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19211 Uichanco-Reproduction in the Aphididw 99 South Africa Myzus persicce Sulz. Moore (1912) Sudan Aphis sorghi Theob. Vuillet (1914) Sudan, Anglo-Egyptian
Aphis sorghi Theob. . Theobald (1904)
United States :-
California
Florida
Indiana3
Texas
Virginia3
South Carolina
Southern U. S.4
Washington
Aphis avenas Fabr. Ewing (1916)
Macrosiphum rosce Ream. Russell (19 14)
Aphis brassicce L. Quaintance
(Herrick, 1911)
Macrosiphum granarium
Kirby Phillips (1916)
Aphis pseudo brassicce
Davis Paddock (191.5)
Macrosiphzm granarium
Kirby Phillips (19 16)
Callipterus trifolii
Monell Davis (1914)
Toxoptera graminu,m Bond.
Webster and Phillips (1913)
Pemphigus bet03 Doane Doane (1900)
Supplementary to the above data may be cited here the other paper of van der Goot (1917, p. 2), who, after three years of bio- logical and taxonomic work on the Aphididse in Java, reported that he had never found amphigonous individuals in that country- not even at high mountain elevations where the temperature falls to the freezing point at night. Likewise, several years of casual observation and collecting in the Philippine Islands by me failed to disclose the male and oviparous-female forms. So far as I am aware, the occurrence of amphigonous aphid individuals in any other tropical country has not been definitely reported. It should be borne in mind, however, that failure to discover such forms in a given locality does not prove their non-existence there; although, when workers carrying on investigation for years in tropical re- gions report their inability to find amphigonous forms in any season of the year, there is some ground for suspicion that partheno- Amphigonous individuals and winter eggs of that species also found in this locality.
South of the 35th parallel.




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100 Psyche [June
genetic reproduction is continuous and uninterrupted, at least among the more common species of tropical aphids. If such a
condition does obtain, the most reasonable explanation appears to be that in the tropics conditions are relatively more uniformly favorable to the aphids, both with respect to the climate and the nutritive factors in the environment.
Patch (192,0), on the other hand, in a very interesting general treatise on the life cycle of aphids, remarks that "in tropical cli- mates experiencing a wet and a dry season gamogenetic [amphigo- nous] eggs are produced to tide over the period of famine"; but unfortunately she does not cite any specific evidence or authority to support her thesis. It is not improbable, however, that some rare cases of amphigonous forms might occur in those tropical countries where, as she suggests, the year is divided between a wet and a dry season: in view of the fact that in the height of the Such a condition obtains in the Philippine Islands, Java, and many other countries of the tropics of both hemispheres, hot, dry season, when conditions are less favorable for many living organisms, certain species of insects are known to assume a resting state, presumably corresponding to hibernation in temperate cli- mates.
In temperate countries, as a rule, reproduction of aphids by par- thenogenesis is continuous during the milder seasons of the year, and amphigonous forms do not appear until the onset of the fall, when the low temperatures begin to affect the insects unfavorably. Exceptions to this generality are determined by the modification of the climate in a given region through the agency of various fac- tors, such, for instance, as the prevailing winds blowing from the ocean and causing the summers to be ((more moderate and the winters milder" on the Pacific Coast of the United States than in regions situated at similar latitudes in the interior and on the Eastern C~ast.~ The aphids in the former locality are thus sub- jected to comparatively more favorable conditions throughout the year and, consequently, they rarely, if ever, undergo heterogony (Ewing, 1916, and Swain, 1919, p. 8).
R. DeC, Ward. 1918.
Climate Considered Especially in Relation to Man, second edition, revised: 44. xvi+380 pp. G. Putnam's Sons, New York and London.




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3 9211 Uichanco-Reproduction in the Aphididce 101 It may be inferred from the foregoing considerations that winter, with its attendant low temperatures and inadequate food supply, plays a very important, if not an exclusive, part in the determination of amphigony. Amphigony correspondingly be- comes of less frequent occurrence as winter becomes less severe in a locality, until in tropical regions where the temperature and, incidentally, the food supply also are more equably maintained throughout the year, amphigonous froms occur very rarely, if at all. In this connection, it is interesting to note that, although heter- ogony has been brought about evidently as an adaptation to un- favorable environmental factors, the effects of low temperatures do not seem to be a necessary immediate 'stimulus in the production of amphigonous individuals and that aphid strains normally under- going heterogony may continue to produce an amphigonous gener- ation and lay eggs at least in the fall following the spring of the same year in which the insects from the open are taken into the greenhouse. The following experimental evidence, which has led me tentatively to arrive at this conclusion, is based on preliminary observations, which will have to be confirmed by further investiga- tions under more adequate control :
In July, 1920, seedlings of Tanacetum vzdgare Linnaeus were transferred to the greenhouse of the Bussey Institution. At about the middle of August, after the plants had been well started, they were inoculated with their common aphid pest, Macrosiphum tan- aceti Linnaeus. The host plants appeared to grow normally and the aphids continually reproduced parthenogenetically until about the end of October, 1920. In October the temperature outdoors began to fall every now and then, especially at night, and at this time the greenhouse was kept heated to a11 average of about 65OF. This temperature had been observed previously by Ewing (1916) to be the optimum for aphids, in that it is least stimulating to the pro- duction of wings in the viviparous forms; and this behavior may be interpreted as an indication that the insects were under more favorable environmental conditions than the individuals which showed greater tendency to produce wings when subjected to other



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102 Psyche [June
temperatures. In spite of the precautions observed in the present experiments, at about the end of October and the first week of November, 1920, when the tansy plants were still in apparent!;' good condition, the aphids produced amphigonous individuals and laid winter eggs, in the same manner as the insects of identical species which were exposed to the more adverse weather conditions in their natural environment outdoors.
In the same greenhouse were a number of plants of Nicotiana alata Otto et Fink. These had been observed by me since Sep- tember, 1919, to be infested with Myzus persicce Sulzer. The insects, which are a common greenhouse pest in this part of the country, were probably descendants of parents that had been asso- ciated with greenhouse plants for a long period of time, and thus had been continually protected for generations from the drastic effects of winter. 'These individuals of .iUyzus persicce in the green- house, which were under the influence of the same temprature con- ditions as the tansy aphids, continuously reproduced parthenoge- netically throughout the winter of 1919-1920 and of 1920-1921. At the time the above experiments were conducted, there was no facility available for a more accurate regulation of temperature and other conditions. The simultaneous presence in the same place of parthenogenetic forms of Myzus pwsicce, however, tends to eliminate to some extent the possibility of adverse conditions exist- ing in the greenhouse and bringing about amphigony in Macro- siphurn tanaceti. Another difficulty in the present experiments was that I had no opportunity to work with material of identical species for control. However, under natural conditions outdoors in this locality, both Myzus persicce an8d Macrosiphum tanaceti be- gin to produce winter eggs at about the same time of the year, in the fall.
In apparent contradiction to my findings on Macrosiphum tana- ceti are the results reported by Slingerland (1893) in his work on Myzus achyrantes Monell, which species is considered by Gillette and Taylor (1908) as a synonym of M. persicce. On April 2, 1890, Slingerland isolated the nymph of a wingless, partheno- genetic female, and within two years and ten months from that date he succeeded in raising from his material sixty-two succes- sive generations of parthenogenetic individuals. The work was done



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19211 Uichanco--Reproduction in the A phididce 103 at the Cornell Agricultural Experiment Station, at which locality winter is more or less severe, and this species is known, under natural conditions, to undergo an alternation of generations during the year. It should be noted, however, that Myzus achyrantes is a common pest of the greenhouses in Ithaca, as Slingerland him- self admits in his paper. It is highly probable, although he does not specifically say so, that he obtained his material of this species from stock which had been in existence in the greenhouse for generations, and, as was the case with Myzus persicce in the present experiments, the progenitors of the nymph with which he started his cultures might have come from a strain that had been repro- ducing exclusively by parthenogenesis as a result of prolonged seclusion from the untoward effects of winter. Under these cir- cumstances then, Slingerland's results would tend to corroborate mine, instead of contradicting them. Nor would Ewing's (1916) eighty-seven continuous parthenogenetic generations of Aphis avence disprove my results, in view of the fact that the source of his material was the Pacific Coast of the United States, where this aphid has been known to reproduce normally by parthenogenesis for indefinite periods. This author himself states in his paper that he was unable to find amphigonous forms of the species out- doors during the entire time that his experiments were in progress. The results reported by earlier investigators, like Bonnet (1745) in France, on nineteen continuous parthenogenetic generations of Aphis sambuci Linnseus, and Kyber (1815) in Germany on parthe- nogenetic reproduction during a four-year period by Macrosiphum rosce Linnasus ( =Sip?~onophora rosa Koch) and Myzus persicce Sulzer ( =Rhophaloslphum dianthi Schrank) , may have to be classed in the same category as Ewing's or Slingerland's, for the reason that (1) at least one species, Myzus persicce, as I have stated, is a common greenhouse pest, and (2) the material with which they worked might have come from stocks which, in their respective localities, had been reproducing outdoors normally by parthenogenesis throughout the year, cases of which have been reported lately from both countries ( Gaumont, 1913, and Borner, 1914). In this event, their results would not tend to contradict mine.
It would not be safe, on the basis of the foregoing evidence, to



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[June
formulate definite conclusions.
The present observations, how..
ever, suggest the following preliminary deductions : 1.
Heterogony in certain aphids of temperate climates has probably become a rhythmic process, occurring regularly at definite periods in their yearly cycle of generations and independently, for a period at least, of the immediate stimulus brought about by adverse temperature conditions.
8.
Amphigonous reproduction in these aphids, although evi- dently maintained as an adaptation to, and under the influence of adverse climatic conditions, continues to occur at these definite cyclical intervals for some time after the causative factors have been removed.
The foregoing views find additional support in the fact that in nature in temperate climates amphigonous and parthenogenetic individuals of identical species in the same locality and feeding on the same parts of a host plant live side by side for considerable periods of time, even weeks, 'on the onset of the fall. I have noticed such a condition in Boston, and other workers have observed it elsewhere. This failure of all the individuals to respond simul- taneously in the same manner to a given condition of the environ- ment tends to show that environmental factors do not furnish the immediate or adequate stimulus in the determination of amphigony. In 1907 Tannreuther (1907), in a paper on Melamoxanthus sali- cis Weed, M. salicicola Thomas, and several other species of aphids, announced somewhat similar views. He said in part that "exter- nal conditions, whether severe or normal, would not bring about the production of sexual generation before a definite number of par- thenogenetic generations had intervened." He further noted, after two seasons of experimentation, that "if a stem mother and off- spring were kept in favorable conditions in the greenhouse on the same species of hosts as out of doors, the time and length of period for each succeeding generation was approximately the same as out of doors, and that in both instances the sexual females and males appeared after the intervention of six parthenogenetic gener- ations."
It may be necessary, at this juncture, to call particular atten- tion to the fact that in the foregoing discussion the role of tem- perature in influencing the form of aphid reproduction is by no



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19211 Uichaftco-Reprodwtion in the Aphid& 105 ems belittled. As has been stated above, adverse temperature nditiom evidently play a very important part in the determina- on of amphigony, and a uniformly mild temperature ia appar- ently conducive to an indefinite mahtainance of p~irthenogenetic me yearly cycle of generatiom migh1
method of reproduction to one of COD,
have no data bearing on the reactiox
.L -*..-*,Y-.-* ---L:-" ----- I-- I.+ --ax-- reproduction. The point suggeeted, however, is that the effect of continuous subjection of an aphid strain which normally undergoes heterogony to mild temperature does not bediately become mani- fest. But this fact does not preclude the possibility that the cmmi- lative effects on more than i
I;
bring about a change in the ,
tinuow parthenogenesis. I I
of the greenhouse aphid8 whikii icpiuuuut; uunnuuuu~iy my prwsuu- genesis to the adverse weather conditions outdoors during the fall and winter montha.
I am not prepared to discuss from personal observations the rela- tion between scarcity of food and "the determination of arnphigony. There ia apparently nothing in the litehture which touches this mbject, except the statement of Tmmenthe~ (1907) that %bun- dance or scarciv of food is not a factor k determining the sex in the case of the aphida,"foor which, however, he presents no concrete experimental evidence. Of eome possible interest in con- 1
section with this prollem are the experiments by Gregory (1917) who found that by subjecting parthenogenetic indi-rifinals of Mawo periods of starvation, in certain cases carrying her experiments to the maximum possible points without killing the insects, she could induce the production from apterous mothers of alate offspring, anhide were invariably parthenoiw&c ancl"tha< the production amphigonous indhicluals -was not artificially induced by the "te merit. It is to be regretted that ehe did not carry her experime . . = .. 7. 3 . m, a, 0,
rougn tlw succeeding generations aner me momer; ana z nestion now arises as to whether the production of amphigono dividnBlfi is induced only bs the successive and ci-imlative effe " "
of starvation on several generatiom of parthenogenetic indi~duala. One "point i8 suggested by these experiments, and that is that, as



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106 Psyche [June
in the case of temperature conditions, if the quantity of avail- able food has any tendency at all to change the method of repro- duction in aphids, the effect does not become apparent immediately. This question, however, needs further investigation before very definite conclusions can be reached.
1. The sexual type of reproduction is the only one known to occur in the Aphididse. This process takes three forms in this family: (a) amphigony; (b) parthenogenesis; and (c) psedo- genesis.
2.
Amphigony is considered as the more primitive method in insects. Parthenogenesis has practically supplanted it in aphid reproduction.
3. Parthenogenesis in aphids is apparently continuous and un- interrupted under favorable environment a1 conditions, amphigon y occurring only under ttte influence of low temperatures and, as certain authors claim, inadequate food supply. 4.
Aphids in tropical and other warm climates appear to have the tendency to reproduce exclusively by parthenogenesis. The same condition apparently obtains among greenhouse aphids in tem- perate climates.
*,
5. Aphids in colder climates undergo heterogony as an adap- tation to adverse environmental conditions. In certain species, the appearance of the amphigonous generation seems to be a rhyth- mic process, which continues to occur at definite cyclical intervals for some-time after the influence of low temperature has been eliminated.
6.
Nothing very definite is known about the relation of food and heterogony. If the quantity of food has any influence at all on the determination of amphigony in a parthenogenetic mother, the 'effect does not become manifest in the immediate offspring. LITERATURE CITED.
BAKER, A. C.
1920. Generic classification of the hemipterous family Aphi- did=. U. S. Department of Agriculture Bull. No. 826. 109 pp. 16 pi.




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19211 Uichanco-Reproduction in the Aphididee 107 1745. Trait,6 dJins6ctologie ou observations sur quelques espkces de vers d'eau douce et sur les pucerons. Durand, Paris. 1914. Blattlausstudien. Abhandl. herausgeg. vom naturwis- sensch. Verein zu Bremen 23 : 164-184.
1875. Monograph of the British Aphides 1. iv+194 pp. PI. 1-38. Ray Society, London.
1882. Ditto, 3. 184 pp. PI. 87-114. Ray Society, London. DAVIS, J. J.
1908. A secondary sexual character of the Aphididse. Canad. Entom. 40: 283-285; 348.
1 pi.
1914. The yellow clover Aphis. U. S. Department of Agricul- ture, Bureau of Entomology, technical series, Bull. No, 25: 17-40. 1 pi. 6 fig. ,
DOANE, R. W.
1900.
A new sugar beet pest and other insects attacking beet. Washington Agric. Exp. Sta. Bull. 'So. 42. 14 pp. 1916. Eighty-seven generations in a parthenogenetic pure line of Aphis avena Fabricius. Bid. Bull. 31 : 53-112. 19 fig-
GAUMONT, L.
1913. Contribution a l96tude de la biologie de puceron noir de la betterave. C. R. de 19Acad. des Sciences, Paris, 157: 1092-1094.
GILLETTE, C. P., AND E. P. TAYLOR
1908. A few orchard plant lice.
Bull. Colo. Agr. Exp. Sta.
No. 133 : 32.
GOOT, P. VAN DER
1915. Beitrage zur Kenntnis der hollandischen Blattlause. Eine morphologisch-systematische Studie. 600 pp. Haarlem und Berlin.
1917. Znr Kenntnis der Blattlause Java's. Contributions la Fauna des Indes Nberlandaises 1 : 1-301. 52 fig.




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108 1's ych e [June
1917 The effect of starvation on the wing development of Microsiphum destructor. Biol. Bull. 33 : 296-303. HERRICK, G. W.
1911. The cabbage Aphis (Aphis brassicce) . Jouni. Econ. En- tomology 4: 2l9-2284.
HERTWIG, E., AND J. S. KINGSLEY
1912 A Manual of Zoology. xii+606 pp. 621 fig. Henry Holt and Company, New York.
KYBEE, J. F.
18 15. Einige Erf ahnmgen mid Bemerkuugen uber Blattlauso. Germar's Magazin der Entomologie 1 : 1-39. 1907. The more important insects injurious to Indian agricnl- ture. Memoirs of the Department of Agriculture in India, entomological series, 1 : 11 3-252.
1912. The green peach Aphis (Myzus persic* and its control. Agricultural Journal of the Union of South Africa 4: 419-428.
PADDOCK, F. B.
1915. The turnip louse. Texas Agric. Exp. Sta. Bull. No. 180. 77 pp.
PATCH, E. M.
1920. The life cycle of aphids and coccids. Ann. Entoin. Soc.
America 13 : 156-167.
PHILLIPS, W. J.
1916. Macrosiphum qranarium, the English grain Aphis. Journ. Agric. Research 7 : 463-480. 2 pi. 1 fig. RUSSELL, H. H.
1914. The rose Aphis. I?. S. Department of Agriculture Bull. No. 90. 15 pp. 3 pi. 4 fig.
SIEBOLD, C. T. VON
1839. Uber die inneren Geschlechtswerkzenge der viviparen und oviparen Blattlause. Froriep7s neue Notizen 12: 305- 308.




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19211 Uichanco-Reproduction in the Aphididee 109 SLINGEELAND, M. V.
1893. Some observations upon plant lice. 11. Science 21 : 48-49.
TANNREUTHER, G. W.
1907. History of the germ cells and early embryology of certain aphids. 2001. Jahrb., Abt. Anat. und Ontog., 24: 609- 642. PI. 49-53.
THEOBALD, F. V.
1904. The "Dura" Aphis, or "Asal" Fly. I. Report of the Wellcome Research Laboratories at the Gordon Memorial College, Khartoum : 43-45.
VUILLET, J. ET A.
1914. Les pucerons du sorgho au Soudan fransais. L7Agro- nomie Coloniale, n. s., 2: 137-143; 161-165. WEBSTER, F. M., AND W. J. PHILLIPS.
1912. The spring grain-aphis or "green bug." U. S. Depart- ment of Agriculture, Bureau of Entomology, Bull. No. 110. 153 pp. 9 pi. 48 text-fig.




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