experimental technique for measuring temperature of an ant

Alternatively I would look around the lab for an infrared thermometer. There exist in the market close focus ones that go down to 6mm in close focus option ( so as not to advertise, google space accurate infrared thermometers microscopes where I found the number in a one of the first hits).

I would choose a large ant, or attract more by a spot of honey and measure their average temperature.

Edit I think finally the answer is like the egg of Colombus,or the Gordian knot) as we say in Greece. Think, do ants have temperature regulator mechanisms?

The egg of Colombus parable : Colombus was sitting at a table and a bowl of boiled eggs came in. All the men around the table played a game of trying to balance the eggs on their narrow end. Colombus said: "what nonsence". and he sat the egg down on its end, breaking it and solving the geometry. :)

Put ant in small volume of cold water, maybe 5ml. Measure the final temperature of water using some thermometer and solve for temperature of ant using the thermal equilibrium equations.

$$m_\text{ant}c_\text{ant}T_\text{ant} + m_\text{water}c_\text{water}T_\text{water} = m_\text{(ant+water)}T_\text{final}(c_\text{water}+c_\text{ant})$$

$$T_\text{ant} = \frac{m_\text{(ant+water)}T_\text{final}(c_\text{water}+c_\text{ant}) - m_\text{water}c_\text{water}T_\text{water}}{m_\text{ant}c_\text{ant}}$$

Note that you need to measure the mass of the ant and water separately before the experiment. Also, choose a volume of water big enough so that you can use your thermometer. To increase precision, choose a temperature of water that is 'more different' than the temperature of the ant, for example choose water that is much cooler than the ant. Also, conduct the experiment in a short time frame to minimize thermal equilibrium with environment.