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Many ot the data here presented were brought together during the several weeks I had the pleasure of spending at the Bussey Institution for Applied Biology of Harvard University. I wish to thank President Henry Fairfield Osborn and the authorities of The American Museum of Natural History for the liberal manner in which I have been able to carry on this work. I am also under great obligations to Professors Wm. M. Wheeler and I. W. Bailey, of Harvard University, for their many suggestions and criticisms during my stay at the Bussey Institution. The interest they have shown in the work has been a steady encouragement and their advice invaluable.
1. Various Relations Between Ants And Vegetation
The question whether ants are, broadly speaking, noxious or beneficial insects is still debated by agriculturists and economic entomologists.
While it is believed on the one hand that ants attack and mine only sick and decaying plants, especially decaying roots, on the other hand it is claimed that health)' plants, which show no trace of disease, are also assailed by ants. In any case further exact observations concerning the relation of ants with plants will be needed in order to clear up this problem. The elucidation of the question of the direct noxiousness of ants to plants is the more desirable, since we possess in the ants partly a welcome help against other animal enemies of culture-plants, which they pursue and destroy. It is therefore necessary that we learn more in detail whether their harmfulness outweighs their utility or vice versa. In general one can perhaps say that, judging from statements which have been made thus far, their noxiousness to plants, by attacking roots, stems or branches, is not very great. (G. Aulmann and W. La Baume, 1912, p. 61.)
In their recent study on the feeding habits of ants, Wheeler and Bailey (1920, p. 236) have pointed out that one reason why the economic importance of many common ants remains so dubious or ambiguous is the lack of precise information with regard to the quality and quantity of their food, especially in the larval stage. These authors have shown, for instance, that ants cany on their bodies and in the food-pellets of their infrabuccal pockets an extraordinary number and variety of fungus spores and bacteria. It is, therefore, quite possible that these insects have a great but hitherto only vaguely apprehended importance a^ carriers ol the germs of certain plant, animal and human diseases. Tha> ants are active carriers of pathogenic micro-organisms has been further suggested by Darling (1913), Wheeler (1914), Studhalter and Huggles (1915), Grabham (1918), and Bailey (1920).
The leaf-cutting ants of the tribe Attini, so abundant in tropical and subtropical America, are decidedly destructive to the vegetation and are rightly considered one of the worst pests to South American agriculture. Accounts of their depredations are found in practically all narratives of South American travellers. Though they attack many of the native herbs, shrubs, and trees, they often show a predilection for cultivated plants. It is no uncommon thing to find the salivas, Atta cephalotes (Linnaeus), so numerous in certain spots that the planters are forced to abandon their fields. Speaking of the ants in the Brazilian coffee districts, Van Delden (1885, pp. 297-298) writes: "The enemy most dreaded in the fazendas (plantations) is indubitably the sauva, or tana-jura, a dark-brown ant , two centimeters long, which undermines the ground by digging extensive passages and dens in all directions. It attacks all sorts of trees, the coffee-shrub among others, but has a decided preference for the orange and citron trees in the coffee gardens." H. W. Bates (1863) and others have noted that these ants often become troublesome to the inhabitants because of their habit of plundering the stores of provisions in houses at night.
The Attini are not represented in the Old World tropics, but possibly ants of other groups have developed similar habits there, though on a smaller scale. G. Aulmann (1912, p. 156) and Moorstatt (1914) mention that a leaf-cutting ant was observed in German East Africa at times causing considerable damage to cotton plants. The specific identity of this ant has not been ascertained, but it probably belonged to the genus Messor, which is known to collect pieces of grass in addition to seeds and grain (see Sjostedt's observation quoted below, p. 359). King (1911) also notes that Messor barbarus (Linnaeus), at Khartum, damages garden plants by biting off and carrying away the leaves, and adds that in cotton fields the sites of their nests are marked by bare patches devoid of vegetation. What use these ants make of the vegetable matter thus carried into their nests has not been investigated.
There are a few other cases on record of ants directly destroying living parts of plants. It is generally known that certain ants will injure buds and fruit in order to feed on the exuding sap (see Muller-Thurgau, 1892, pp. 134-135). Forel (1885, p. 338) mentions instances of Tetra'morium cxspitum (Linnaeus) attacking young roots of healthy sugarbeets at Vaux, Switzerland, many of the plants dying from the injuries received. J. Perez (1906, pp. xxxii-xxxiv) records the havoc played by the same ant on the tubers of potato, near Bordeaux, more or less deep cavities being excavated and many young plants killed; T. cxspitum was also found burrowing superficial galleries in the stems of living potato plants and attacking the roots of young cabbage and carrot.1 In North America, Solenopsis geminata (Fabricius) and S. molesta (Say) often do injury to the soft parts of planted seeds, and the former also to strawberries (Webster, 1890) and other fruit. S. molesta has proved very injurious in gardens and fields; the chief damage is done to seeds of sorghum and corn, which are hollowed out undoubtedly for the purpose of extracting the oils (McColloch and Hayes, 1916; Hayes, 1920). According to Green (1900a) and G. R. Dutt (1912, p. 247), the Indian Dorylus orierdalis Westwood is mainly or exclusively herbivorous, feeding on the bark of trees and the healthy tubers of plants, a habit the more remarkable since the majority ot Dorylinae are highly carnivorous. In Cameroon, certain ants have been seen attacking the fruits of cacaotrees: Camponotus maculatus subspecies brutus (Forel) gnaws the base of fruit-stalks where they are inserted into the trunk, licking up the sap at the wound, causing the fruits to drop off or dry; Crematogaster africana variety ivinkleri (Forel) gnaws away the skin of the cacao-fruit, often almost completely; while Camponotus acvapimensis Mayr and CEcophylla longinoda (Latreille) are accused of the same evil, though they cause but little damage (H. Winkler, 1905, pp. 129-137).
The greatest harm to the vegetation is undoubtedly done indirectly, both in tropical and temperate regions, by a host of species of ants that have a pronounced fondness for pasturing and guarding plant lice, scale insects, tree-hoppers, and other plant bugs on roots, stems, and foliage; all these Hemiptera suck the juices of plants, and their protection by the ants must, therefore, be regarded as pernicious. The "milking" habit among ants seems to be of very frequent occurrence, evidently because it offers so many advantages over direct feeding on plant-juices. Not only is the food supply much more abundant at any one time and within easier reach, but, in addition, the plant saps undergo chemical changes in the digestive tract of the Hemiptera, whose anal secretion, on which the ants feed, therefore contains a great amount of invert-sugar, instead, of the much diluted cane-sugar of the plant. Many of the aphids attended by ants have undergone adaptive modifications of structure and behavior which show that their relations with ants have become of a mutualistic nature, and it is probable that the same will be found true for some of the ant-attended coccids and membracids of the tropics.
iThi.s habit of Tetramorium csespitum in attacking subterranean parts of plants was known to Linnaeus, since he adds to the original description of this ant ('Syst. Nat.,'Ed. 10,1,1758, p. 581): "Habitat in Europa ? tuberibus." It is rather surprising that injuries by this ant have been so little noticed in later times. Concerning ants noxious in gardens, see also F. Heim (1894), Andersson (1901), and Cooley (1003).
Indeed, the association between phytophagous Hemiptera and ants offers a typical illustration of symbiosis in the strict sense, advantageous to both parties. The benefit that accrues to the ants has been explained above and needs no further comment; that derived by the Hemiptera, however, is of a more complex nature. It is obvious that the ants protect the plant bugs by driving away coccinellid beetles, ichneumon flies, and other enemies. In the case of aphids and coccids the ants frequently build tents or cowsheds over these insects, which thus continue to suck the juices of the plant while being "milked" by the ants and are, at the same time, protected from their enemies, from alien ants, and intemperies, and prevented from escaping to other plants.
The tent-building habit was discovered by P. Huber (1810, pp. 198-201) for Lasius niger (Linnaeus) in Europe, and Forel (1874, pp. 204-205 and 420 422) gives an interesting account of it in his classical 'Ants of Switzerland.' Lasius 7iiger has similar habits in North America (Wheeler, 19116) and Japan (Stopes and Hewitt, 1909, pp. 1-6). This ant builds its tents of detritus or wood-fibres; while, according to Forel, certain species of Myrmic.a enclose their aphids in earthen cells, which communicate with the ground nest by means of covered galleries. Wheeler (loc. cit.) has described in detail the tent-building of the North American Crernatogaster lineolata (Say) and I have found that several African members of this genus which attend coccids have similar habits. Certain North American species of Lasius (L. flavus, L. niger, and the species of the subgenus Acanlhomyops) which live to a very large extent or exclusively on the excrement ot root-aphids and coccids, remain throughout the year the constant companions of the lice, even hoarding in their nests during winter the eggs or the wingless, agamic form of the aphids and the fertile females of the scale insects. Forbes (1896), Webster (1907). and others have shown that the common North American Lasius niger variety americanus Emery guards the eggs of the corn root aphid (Aphis maidi-radicis Forbes) throughout the winter, shifting them about, as it does its own young, to accommodate them to changes of weather and moisture. In spring, the young lice, on hatching from these eggs, are conveyed by the ants during fair weather to the roots of various weeds, being taken back to the burrows in bad weather or on cold nights. After the corn plants have started to grow, the young root lice, all of which belong to the wingless, agamic form, are transferred from the weeds to the roots of young corn, where they are tended throughout the spring and summer. It would thus appear that, without the aid of the little brown ant, this aphid is unable to reach the corn plants.
Still more surprising is Lubbock's observation that Lasius flavus cares in a similar manner for the eggs of certain aphids on the aerial portions of plants.
The eggs are laid early in October on the food-plant of the insect. They are of no direct use to the ants, yet they are not left where they are laid, where they would be exposed to the severity of the weather and to innumerable dangers, but brought into their nests by the ants, and tended by them with the utmost care through the long winter months until the following March, when the young ones are brought out and again placed on the young shoots of the daisy. This seems to me a most remarkable case of prudence. Our ants may not perhaps lay up food for the winter, but they do more, for they keep during six months the eggs which will enable them to procure food during the following summer. (Lubbock, 1880, p. 184.)
In temperate regions the honeydew (or sugary excrement) secreted by aphids from the posterior end of the alimentary canal is eagerly sought for by many of the common Myrmicinae, Dolichoderinae, and FormicinaV those attending root lice being especially harmful to the vegetation for the reasons mentioned above.2 Certain tropical ants also nurse root-aphids. In Java, Acropyga acutiventris Roger may thus become a serious pest to coffee plantations, and, according to Forel, various species of Rhizomyrma attend root lice in South America and New Guinea (K. Escherich, 19116, p. 227, footnote). In the tropics, however, aphids are far less common than in colder climes and are there replaced as ant "cows" by various Coccidae, Membracidse, Fulgoridae, Cicadellidae (Jassidae), and Psyllidae, certain members of these families being occasionally attended by ants even in North America and Europe.
The relations between various species of tree-hoppers and certain ants have been recently reviewed by Funkhouser in his 'Biology of the Membracidae of the Cayuga Lake Basin' (1917, pp. 399-404), to which the student is referred for further details. Funkhouser comments on the number of unsolved problems in connection with this subject.
One of the first of these questions is suggested by the fact that some of the species are attended by ants while others are unattended although there are apparently no physiological or anatomical differences to cause the distinction. Another question arises from the fact that certain species attended locally have never been reported as being attended in other parts of the country, while on the other hand some of the species that are never attended in this basin are always attended in other localities. Again, certain species that the ants ignore in this basin are represented by closely related species in other regions and these exotic forms—often of the same genus ard very near systematically—are well attended.
'It would appear that these trophobiotic habits are of great antiquity among ants, dating as far back as the Tertiary. Wheeler (1914. p. 21) found a block of Baltic amber containing a number of workers of Iridomyrmex gopperti (Mayr) together with a lot of their aphid wards.
2See the publications of S. A. Forbes on the corn root aphid, listed in the bibliography; also Garman's (1895) account of the bean root louse.