COMMUNAL SILK-SPINNING by LARVAE of DENDROMYRMEX TREE-ANTS (HYMENOPTERA : FORMICIDAE) Edward O
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Insectes Sociaux, Paris Masson, Paris, 1981 1981, Volume 28, n ~ 2, pp. 182-190 COMMUNAL SILK-SPINNING BY LARVAE OF DENDROMYRMEX TREE-ANTS (HYMENOPTERA : FORMICIDAE) Edward O. WILSON Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138, U.S.A. Rein le 24 septembre 1979. Accept6 le 30 novembre 1979. SUMMARY Dendromyrmex charti[ex and D. [abricii are weaver ants: the mature larvae of these formicine species add silk to the arboreal carton nests. This material strengthens the layers of vegetable fibers, 'which form the bulk of the nests and are contributed by the ~vorkers. The communal siLk spinning is the most primitive yet discovered and may represent the first stage in the evolution of 'weaving. The larvae, rather than the workers that hold them in their mandibles, ma~ke the lateral vceaving movements, and their behavior differs from conventional cocoon spinning only in the orientation of the movements relative to the body. Moreover, larvae often contribute silk in this manner even when unattended by vcorkers. Both male and worker-destined larvae spin commu- nally; whether they contribute equal quantities per capita remains to be determined. Other information on the natural history of Dendromyrmex based on field and laboratory studies is reported: the colonies are polydomous; the workers forage nocturnally and accept a 'wide variety of insect prey. The workers of D. chartilex have unusual genal eyespots and behave in such a way as to resemble polybiine wasps, and it is suggested that they are in fact Batesian mimics of these stinging insects. ZUSAMMENFASS UNG Kommunales Seide-Spinnen (lurch Larven der Dendromyrmex Baumameisen Dendromyrmex chartifex und D. fabricii sind Weberameisen : Die erwachsenen Larven dieser Formicinen-Arten fiigen den arborealen Kartonnestern Seide zu. Dieses Seide- material verst~irkt die aus pflanzlichen Teilchen bestehenden ~V~inde der Nestkammern, die yon Arbeitern gebaut werden. Das kommunale Seide-Spinnen yon Dendromyrmex ist die bis jetzt primitivste Form dieses Verhaltens und kfnnte die erste Stufe in der Evolution des Webens bei Ameisen darstellen. Nicht die Arbeiter, welche die Larven mit den Mandibeln halten, sondern die Larven machen die seitlich gerichteten Spinnbewegungen. Dieses Verhalten unterscheidet sich vom konventionellen Kokon-Spinnen nur in der Often- tierung der Bewegungen in Bezug zum K6rper. Oft produzieren sogar die Larven Seide zum SILK-SPINNING BY LARVAE OF DENDROMYRMEX 183 Nestbau, ohne dabei yon den Arbeitern gehalten zu ~erden. M~innchenlarven und Arbeiterlarven zeigen kommunales Spinnverhalten; ob beide die gleiche Menge Seide per Individuum produzieren muss noch festgestellt vgerden. Weitere Beobachtungen im Freiland und Laboratorium zur Biologic yon Dendro- myrmex xverden berichtet: Die Kolonien sind polydom; die Arbeiter furagieren nachts und nehmen eine Vielfalt von Insekten als Futterbeute an. Die Arbeiter yon D. chartilex haben auffallende Augenflecken und verhalten sich als ob sic Polybiinen-'Wcspen nachahmten. Es wird vermutet, dass es sich bier um eine Batessche Mimikry dieser stechenden Insektcn handelt. INTRODUCTION In several phyletic lines of the ant subfamily Formicina~ the larvae have evolved the habit of diverting silk from the production of their own cocoons to the construction of parts of the colony's nest. These "weaver ants " include both species of the Old World genus Ofcophylla (H/SLLDOBLER and WILSOn, 1977, 1978), many members of the ~videspread Old World genus Polyrhachis (KARAWAJEW, 1929 ; OVER, 1970), and Camponotus senex of Central and South America (FOREL, 1905; and personal observation). According to BARON1 URBAm (1978), silk is incorporated into the earthen nests of some Cuban species of the myrmicine subgenus Leptothorax (Macromischa), but this author is unsure .whether the material is obtained from larvae or from some other source such as spider webs. Since no other myrmicine is known to produce silk, the latter alternative seems more probable. (EcophylIa represents an extreme in communal silk spinning by larvae. The final-instar larvae never manufacture cocoons of their o'wn; all of the silk from their hypertrophied silk glands is donated to the construction of the elaborate silk-and-leaf envelopes that exclusively constitute the nests of these ants. The larvae are used like shuttles by the major 'workers. The ants grip them from above at the middle of their bodies and vceave them from side to side while repeatedly drumming them lightly on the )lead ,with the tips of their antennae. The larvae respond by arching their bodies into a shallow S-shape form and releasing threads of silk from the labial sericteries. Thus 'while the larvae produce the silk, the adult major vcorkers guide the silken threads and in effect control the design of the nest. The larvae of the advanced 'weaver ants therefore cooperate 'with the remainder of the colony by providing a specialized form of labor. They also donate part of the protein synthesized -,vithin their own bodies. They have in effect become a separate caste. It would be of substantial interest to trace the early stages in the evolutiorr of this remarkable speciaIization, and the analysis can perhaps be accomplished by a careful study of the various independently evolved but less advanced ,weaver ants. To this end, I have been able to establish that larvae of two species of the Neotropical 184 E. WILSON tree-ant genus Dendromyrmex regularly contribute silk to the nest construc- tion, but in a simple, generalized manner that appears to be a very early stage in the evolution of communal silk spinning. NATURAL HISTORY OF DENDROMYRMEX Dend, omyrmex is a member of the tribe Camponotini characterized by monomorphism in the 'worker caste and a slight constriction of the occipital region into a neck-like form. According to EMERY (1925), its seven species are concentrated mostly in Brazil and northern South America, but at least two (chartifex, fabricii) range into Panama. WEBER (1944) reports that all three species he examined in the field (apicalis, chartifex, fabricii) in South and Central America form small colonies that construct oblong carton nests on the leaves of a variety of tree species in rain forest. All of the nests encountered by WEBER 'were at a height of 2-1/2 meters or less, but his observations ,were too limited to preclude their occurrence at higher levels in the forest canopy. Colonies are started by one or several queens, which build miniature carton nests in similar forest sites. Data gathered on D. chartifex and D. [abricii in Panama during the present study (see Acknowledgments) confirm these generalizations. They also show that the carton nests usually contain 30 adults or less. None of the Panama nests contained dealate queens. When the units 'were maintained in the laboratory, either no additional eggs were laid (chartifex) or else those laid -- by 'workers -- developed into males (fabricii). It is therefore apparent that the colonies encountered 'were polydomous, and the units collected were colony fragments. My laboratory studies of chartifex and fabricii have yielded other new facts. The workers are exclusively nocturnal; those of at least chartifex were also found to be nocturnal in the field in Panama (R. E. SILBERGLIED, personal communication). They readily accept sugar water, honey, and a wide variety of freshly cut insects, including Nauphceta cockroaches, tipulids, chironomid midges, mosquitoes, earwigs, and a variety of small moths, 'without apparent discrimination. Since the colony fragments thrived on this diet for a period of months, it is concluded that they are general insectivores and will collect honeydew if given the opportunity. No evidence was found of recruitment to food, even when the colonies were hungry and individual foragers had been excited by the discovery of food items too heavy to carry alone. The 'workers of D. chartifex are characterized by large paired yellowish spots on the genae (cheeks), 'which stand out conspicuously against the dark- brow pigment of the remainder of the body. When the colony is disturbed, many of the 'workers line up side by side along the edge of the nest entrance and face outward, 'while bobbing up and dawn. The visual effect in the SILK-SPINNING BY LARVAE OF DENDROMYRMEX 185 relatively weak lighting of the laboratory and forest is striking: to me the genal spots resemble eyes, and the ants resemble small polybiine wasps. I have considered the possibility that the ants are 'wasp mimics. SILBERGLIED (in litt., 5 August 1978) agrees, stating that in Panama " the most likely model would be a Protopolybia species, possibly pumila (?), that builds small, paper nests under leaves. These nests are quite flat and might be mistaken for a Dendromyrmex.,. The only trouble 'with the Protopolybia species I am thinking of is that it is extremely docile. Another possibility, Metapolybia, also builds small, flat nests; it too is docile." But these 'wasps may not be docile to the visually orienting predators that most endanger them, Dissection of nests of chartifex reveals that they contain substantial layers of silk, 'which are laid down both below and above the main carton layers of vegetable fibers. Some silk also appears to be mixed as a minor component ,with the'vegetable fibers. Additional sheets of silk, limited in extent, are extended over openings in the main carton; it is possible that they represent substrata on which additional layers of vegetable fibers are destined to be laid. By far the greatest bulk of the nests consists of vege- table fibers. The silk is a supporting element and also serves to give the carton walls greater strength, a fact that can be ascertained by making small tears through the 'wails at various positions. Laboratory observations quickly established that the silk is produced by the larvae, in behavior patterns that are primitive in comparison to the advanced (Ecophylla 'weaver ants. Because of the light that it might shed on the evolution of communal spinning, this behavior 'was selected for closer study.