Communication with the host ants

The caterpillars of the Alcon blue need to be adopted by a Myrmica ant nest and to be fed and cared for by the worker ants if they are to survive. This means that they must produce signals which persuade the ants into adopting and caring for them, despite the fact that this is against the best interests of the ant colony.

The fourth instar caterpillars of the Alcon blue, like those of most other ant-associated lycaenid butterflies, have several sets of specialised organs for producing such signals. The picture below to the left (A) is a scanning electron micrographs of a fourth instar caterpillar. An area toward the rear end of this caterpillar is shown at B. Two of these types of organ, the Dorsal Nectary Organ and the Pore Cupolae have been artificially coloured on this close-up. A further close-up of two pore cupolae is shown in picture C.

The Dorsal Nectary Organ

This is a large secretory organ found toward the rear end of many lycaenid caterpillars. It most lycaenids that possess it, it produces droplets of a honeydew-like liquid which are eagerly consumed by ants. In the large blue butterflies it only seems to be used during the adoption process, where a droplet of secretion is provided for Myrmica ants which find a caterpillar in the field. In the Alcon blue adoption often occurs without a secretion droplet being produced, and we are currently investigating whether this depends on the species of ant that finds the caterpillar. The composition of the secretion from the Dorsal Nectary Organ is not known for the Alcon blue, but in other lycaenids it contains sugars and sometimes amino acids.

Pore cupolae

These are small secretory organs found scattered over the upper surface of all lycaenid caterpillars. They have a characteristic structure with a flat plate with many small pores in it through which small quantities of the secretion of the gland flow. The pore cupolae are known to produce amino acid solutions in some lycaenids, and they may also produce hydrocarbons which coat the surface of the caterpillar.

Stridulatory organs

The larvae and pupae of lycaenid butterflies are known to produce vibrational signals. There is still controversy as to whether these are signals produced for ants or not.

Although the exact method of vibration production in lycaenid larvae is not known, it seems likely that it is through rubbing together of particular regions of the cuticle which are shaped rather like a washboard.

A region of the intersegmental cuticle of a fourth instar caterpillar of the Alcon blue that may be used for producing vibrational signals is shown to the right

Surface hydrocarbons

Nestmate recognition in ants is thought to be primarily through the hydrocarbons that all the ants within a colony have on their cuticle.

It is likely that the caterpillars of large blue butterflies also use surface hydrocarbons to signal to the ants, and main gain access to the ant colony and its food by mimicking the surface hydrocarbons of the host ants.

Research is currently under way at the University of Copenhagen, in collaboration with the University of Keele, to characterise the surface hydrocarbons of the Alcon blue and its host ants.

The picture to the left shows a gas-chromatograph trace of the surface hydrocarbons of an Alcon blue caterpillar.

Researchers at the University of Copenhagen and Aarhus who are currently investigating communication between the Alcon blue butterfly and its host ants are:
Dr. David Nash
Thomas Damm Als