One factor that is very important is the presence of elaiosomes. Quoted from my textbook:

"Many species [e.g., Acacia (wattle) species in Australia] produce a lipid-rich morphological structure, termed aril or elaiosome. Such a structure allows dispersal via ants (Hughes et al. 1994), which transport the seeds to their nest, thus burying the Acacia seeds, safe from fire (O'Dowd & Gill 1985). Cabralea canjerana (cancharana), on the other hand, is a typical bird-dispersed tree in Atlantic forests in southeast Brazil. Ants treat their seeds in different ways, depending on the species. Some ants remove the arillate seeds to their nest, thus reducing seed predation by insects and rodents. Other ants remove the aril on the spot, or cover the seeds before removing the aril. Aril removal greatly facilitates seed germination (Pizo & Oliviera 1998).

Yes, Hans picks out the key factors - also, it's interesting that the association is strong enough for a phasmid species Extatosoma tiaratum (Macleay's spectre) to have evolved seed-mimic eggs with a elaiosome-like structure. This is eaten by ants and the egg dumped in the nest waste-heap. They hatch there and the 1st instar nymphs are unsurprisingly ant-mimics.

Interesting comment, David. Do you have a reference for this?

Most of the info seems to come from insect-keepers etc (and so is anecdotal though I believe accurate) or in the hard-to-find 'grey' literature like the Phasmid Study Group newsletter, but there is this:

Key, K.H.L. (1991). Phasmatodea (Stick-insects). pp. 394-404 in CSIRO (ed.) The Insects of Australia. A textbook for students and research workers. Melbourne: Melbourne University Press, Vol. 1, 2nd Edn.

Also:

Korboot, K. (1961). Observations on the life histories of the stick insects Acrophylla tesselata Gray and Extatosoma tiaratum Macleay. University of Queensland Papers (Entomology) 1: 159-170.

Thanks David. I'll try to get hold of those references. I definitely learned something here.

It would appear that you and I are more interested in exploring answers to the question that was asked than the person who entered that question here on RG. Then again, that appears to be rather common, in my experience on RG.

:-)

I managed to download that last paper, and thus saved the two shillings it would have cost me if I had got it when I was 10 years old.

You can read the textbook's Deborah Gordon and others article and text of this researcher, e.g., Ants at Work: How an Insect Society is Organized, Ant Encounters:

Interaction Networks and Colony Behavior (http://www.stanford.edu/~dmgordon/)