The Biogeography of Army ants (g.Dorylinae)
by Zachary Gifford student in Geography 316, sp 99
( Gotwald, 1995 )
Species: dorylus, eciton, aenictus
( Gotwald )
Description of Species: It looks like a graceful undulating ribbon. One of the fist people to draw attention to them was a man named Bates in 1863 what follows is his notes " Wherever they pass they through the rest of the animal world in a state of alarm. They steam along the ground and climb any lower trees searching every leaf to its apex. When the booty is plentiful they concentrate all of their forces upon it, the dense phalanx of shining and quickly moving bodies swarm. As it spreads over the surface they look like a flood of dark red liquid. They soon penetrate every part f the confused heap, and then, gathering together again in marching order, onward they move. They tear their victims in pieces for facility of carriage. The margins of the phalanx spread out at times like a cloud of skirmishers from the flanks of an army". The designation "army ants" although an analogy fits well, these creatures given to carrying out attacks en masse.
Habitat: The army ants as figure 2 shows have a worldwide distribution, with representatives in each hemisphere roughly between 45 degrees north and 45 degrees south. They are found in most tropical and subtropical parts of the world with some notable exceptions. In the New World Chile, the volcanic islands of the Caribbean, mountain heights, and certain extensive desert and swampy areas. In the Old World the Sahara, Afghanistan, and desert areas, as well as the afore mentioned mountainous and swampy regions (Schneirla 1971).
Evolution: Now let us briefly consider the
origin of army ants. It is generally agreed that ants, as completely socialized
insects, arose from an archaic stock resembling a modern solitary wasp.
Emery (1985), on taxonomic grounds, favored a mutillidlike type of ancestor
and dated the origin to the early Cretaceous times i.e.. Roughly 130 million
years ago. Others have suggested on ecological and other grounds that the
time is more around the Triassic Period, the thought was that the insects
could have flourished in the hot dry uplands then very prevalent. They
might have thrived on a diet of the roach like insects then on the increase
(Schneirla 1971). Recently the discussion has taken a more realistic turn
with the finding of two amber embedded fossil specimens. The two workers
probably from the same colony are dated around 100 million years ago. The
workers combine wasp like characteristics with such other features as a
well formed petiolar node, which is defiantly characteristic of ants forming
small colonies of interdependent, hence socialized individuals (Gotwald
1995). Although existing ants live in colonies, a minority of them are
considered very primitive, suggesting what early forms were like. From
these, one gathers that mechanisms basic to group unity and group function
must have evolved slowly at first. Through the great advantage of group
action in getting food and in maintaining a collective shelter, ancestral
ants presumably advanced steadily in evolving social functions and behavior.
From insects of this type, possibly very early in their evolution, may
have sprung the predatory stock ancestral to army ants (Schneirla 1971).
It is also possible to speculate about the evolution of behavior in the origin of the army ant adaptive syndrome. A working hypothesis compromising major adaptive steps in the elaboration of army ant behavior is given by Gotwald, I think it addresses the last of the major points of this paper.
1. Group recruitment raiding develops; permitting specialized feeding on other socialized insects. That is, group raids are led by scouts to prey discovered by the scouts.
2. Group raids are initiated autonomously without the recruiting activities of the scouts. These rather more massive raids allow specialized feeding on large anthropods and other social insects and permit a larger part of the trophic field to be searched. This type of raiding occurs without frequent migrations.
3. Nomadism either develops concurrently with group raiding behavior or is added shortly afterward. Emigrations to new nest sites allow a colony to shift to new trophophoric fields with more abundant prey. This combination exemplifies army ant behavior.
4. As group perdition becomes more efficient, large colony size is possible.
5. Diet may expand to include other smaller and non-social anthropods, some small vertebrates, and vegetable matter. Colonies become quite large.
Distribution: Early Tertiary origin for the Dorylinae. Schneirla postulated a similar origin and explanation for the distribution of the ants. The end of the Cretaceous already separated the three tropical areas in which the true army ants are the army ants as figure 2 shows have a worldwide distribution, with representatives in each hemisphere roughly between 45 degrees north and 45 degrees south. They are found in most tropical and subtropical parts of the world with some notable exceptions. In the New World Chile, the volcanic islands of the Caribbean, mountain heights, and certain extensive desert and swampy areas. In the Old World the Sahara, Afghanistan, and desert areas, as well as the afore mentioned mountainous and swampy regions (Schneirla 1971). Given certain assumptions, present day distribution pattern of army ants can be correlated with the movement of continents. Based on the estimated antiquity of the ants in general and diversity of the Oligocene (36 million to 25 million years before the present) ant fauna. Gotwald hypothesized a late Cretaceous, possibly currently found separated by substantial bodies of water. This suggests that the "true army ants" underwent convergent evolution at three separate tropical loci: the Ecitoninae in South America, Dory's in Africa, and Aenictus in Laurasia. The probability is low that the army ants, poor disperse, originated in a single place and distributed throughout the world over ocean barriers. Quite clearly, they are restricted by the fact that the queen is wingless and those new colonies are produced through fission of existing colonies (Gotwald 1995).
Map of Distribution:
( Schneirla 1971 )
Goetsch, Wilhelm, 1969. THE ANTS. University of Michigan Press
Gotwald, William, Jr. 1995. ARMY ANTS THE BIOLOGY OF SOCIAL PREDATION. Cornell University Press.
McCook, Christopher, 1909. ANT COMMUNITIES. Harper & Brothers Publishers, New York and London.
Schneirla,T.C. 1971. ARMY ANTS A STUDY IN SOCIAL ORGANIZATION. Department of Animal Behavior, The American Museum of Natural History, New York.
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