Ants
Homology Weekly: Tentorial Pits
The anterior tentorial pits (arrows) in a Tetraponera aethiops worker (Scanning Electron Micrograph, Roberto Keller/AMNH)
The head of an ant in frontal view has a couple of holes usually located in the area between the mouth and the place where the antennae are inserted. These holes look intriguing from the outside– Are they part of a sensing organ? Do they secrete a special chemical signal or defense substance through them? Are they use for breeding? The answer is more mundane than that. As I mentioned in an earlier post, most of what one sees in the outer surface of the arthropod’s exoskeleton does not have an external function, but is rather a symptom of the inside working in these wonderful machines. These particular holes mark the places where the cuticle invaginates to form the internal skeleton of the insect cranium known as the tentorium. The external holes produced by these invaginations are thus termed the tentorial pits.
Monomorium ants in Andalusia
The Alhambra in Granada, Spain.
I recently traveled to Andalusia, in the southern part of the Iberian Peninsula, to meet fellow myrmecologists Christian Peeters, from the Université Pierre et Marie Curie, and Alberto Tinaut, from Universidad de Granada. The reason for my trip was that I am fortunately enough to have been invited to collaborate in one of their ongoing projects studying the native ant species Monomorium algiricum. We set out to collect some colonies of this species as well as some others in the genus.
Homology Weekly: Petiole, Postpetiole and “Tubulation”
An isolated second abdominal segment constitutes the characteristic petiole (blue) in ants. Pachycondyla stigma worker (Scanning Electron Micrograph, Roberto Keller/AMNH)
The easiest way to know you are looking at an ant is to pay attention to its waist: if it consists of one or two nicely isolated segments you can be sure you made a positive identification. The basal condition for the family, common to all ants, is to have the second abdominal segment in the shape of a node or scale and distinctly isolated from the rest of the abdomen to form a petiole (remember that the first abdominal segment is coupled to the thorax as the propodeum). The functional advantage of such novel architecture seems to be an enhanced articulation between body segments, and thus greater mobility for a posterior part of the body that bears the ant’s weapons in the form of a sting or other specialized chemical producing organs like the acidopore.1
› Continue reading
- This post is dedicated to my long time friend and colleague Francisco Vergara-Silva ↩
Homology Weekly: Clypeus
Tetraponera aethiops worker showing the location of the clypeus in green (Scanning Electron Micrograph, Roberto Keller/AMNH)
When looking at an arthropod from our vertebrate perspective it is easy to forget that we are looking right at the animal’s skeleton. While our own vertebrate skeleton consists of a series of internal compact pieces with sponge-like cores that support an external layer of muscles and entrails (all nicely wrapped in skin), the reverse is true for arthropods. The arthropod skeleton consists of a series of external plates and hollow tubes that form enclosed spaces within which the internal musculature system attaches1. One consequence of this peculiar body architecture is that most of what we see on the outer surface of this exoskeleton is but a reflection of what is going on on the inside– minute external pits correspond to places where the cuticle folds in to form internal pillars, and innocent looking shallow furrows on the surface are large internal walls where powerful muscles originate. A simple examination of the exoskeleton, therefore, can tell us a lot about particular functions and consequently about an insect’s behavior. › Continue reading
- The only enclosed cavity formed by the skeleton in vertebrates is the cranium, but there are no muscles inside it. ↩
Homology (Bi)Weekly: Dentiform Labral Setae
Red Hot Chilli Peppers? No, dentiform setae in the labrum of an Onychomyrmex doddi worker (Scanning Electron Micrograph, Roberto Keller/AMNH)
Just as the anterior margin of an ant’s cranium can sometimes be armed with rows of dentiform clypeal setae (that is, especially modified hairs), the lid that closes the insect’s mouth called labrum can bear identical structures. The image above shows two of these specialized teeth-like pieces (in red) flanking an empty broad socket where a third piece used to be inserted.
Homology Weekly: Mouthparts
Frontal part of the head in an Anochetus emarginatus worker, profile view (Scanning Electron Micrograph, Roberto Keller/AMNH)
This image shows the mouthparts of a trap-jaw ant in resting position. The only structures really visible are the prominent elongated mandibles (in yellow) that project forward. The rest of the pieces, laying immediately below, are retracted inside the preoral cavity.
Honeybee or Honey Bee?
In the preface of his 1956 classic Anatomy of the Honey Bee1 the great American entomologist Robert E. Snodgrass explains the book’s title:
First, it must be explained why the name of the bee appears in the title as two words, though “honeybee” is the customary form in the literature of apiculture. Regardless of dictionaries, we have in entomology a rule for insect common names that can be followed. It says: If the insect is what its name implies, write the two words separately; otherwise run them together. Thus we have such names as house fly, blow fly, and robber fly contrasted with dragonfly, caddicefly, and butterfly, because the latter are not flies, just as an aphislion is not a lion and a silverfish is not a fish. The honey bee is an insect and is pre-eminently a bee; “honeybee” is equivalent to “Johnsmith.” [vii]
- Snodgrass, R. E. 1956. Anatomy of the Honey Bee. Cornell University Press. Ithaca, New York. ↩
Homology Weekly: Metapleural Gland
Tapinoma simrothi worker. The rectangle shows the opening location of the left metapleural gland (Scanning Electron Micrograph, Roberto Keller/AMNH)
The metapleural gland is the definitive character of ants. It is unique to the family. Nothing homologous or similar is found anywhere else in insects. Within the tree of life of Hymenoptera, myrmecologists agree that the appearance of this gland provides a good cutting point to marks-out ants as a monophyletic group1. You have it? You are an ant. You don’t? Sorry, you don’t qualify, get the hell out of here lousy wasp2. It is the ultimate ant synapomorphy.
- Grimaldi, D. and D. Agosti (2000). The Oldest Ants are Cretaceous, Not Eocene: Comment. Canadian Entomologist 132(5):691-693. ↩
- Yes, one can insult insects by calling them members of the Order Phthiraptera ↩
Homology Weekly: DNA
At any given position along their DNA sequences, ants may have any of the following four nucleic acid types: A, T, C or G. Unless we are dealing with the mitochondrial genome that is known to have quite a few A+T-rich regions in insects, in which case expect to find just those two types of nucleotides.
Homology Weekly: Arolium
Foot of a Oecophylla smaragdina worker. Pretarsal claws and manubrium in red; arolium in yellow; tarsi in green (Scanning Electron Micrograph, Roberto Keller/AMNH)
This orchid-looking thing is really the foot of an ant. The large unfolded structure in between the powerful pair of claws is the adhesive organ of the foot called arolium (pl. arolia). It is basically a soft membranous bag folded into a suction cup that allows the ant to walk on vertical or upside down smooth surfaces.
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