I was just wondering how would a humanoid like Ant species develop communication. I know how they communicate with each other in nature using pheromones and touch. Would it be possible that this species could evolve more of a verbal type of communication through the mouth?
The basis of all communication is common ground and context.
In addition to a physical inability to make audible sounds, an ants mind would be so far removed from most human concepts that communication on any level would be insanely difficult.
Ants, as you mentioned, communicate predominantly using pheromones and chemical secretions. They are believed to operate as cohesive elements of the greater population, controlled in a strict hierarchy.
About the only common ground we would have with ants would be on topics such as food, water, fight or flight. Probably navigation and giving direction would play a large part in an ant's chemical vocabulary.
We are currently unable to really communicate with whales and dolphins, even though they are much closer to us in terms of evolutionary development than ants.
Unfortunately, I think it would be a major struggle to communicate in any meaningful way with ants.
However, if you anthropomorphise ants to the point that they have a more human mouth, voice box, lungs etc. you might have a chance. Another option might be to communicate by a form of sign language using body parts, antennae.
The main evolutionary pressure to develop complex communication in ants would be for finding food.
Ants, as well as bees, generally find food by having individual foragers moving around at random until they find a food source, then returning to the nest and leading the other ants there. Ants lead each other to food using physical pheromone trails, and that works for them, so they would have little reason to develop more complex communication.
Bees, being flying insects who are unable to lay down physical trails, instead use dances to point out the direction to a food source. These dances, however, tend to be instinctive and inflexible, indicating the direction of the food source relative to the position of the sun, and the distance from the nest. For bees, this suffices to tell them all of the information they need to know, as bees don't really need to navigate a more complex, maze-like environment.
For a true language to develop, there would need to be a need for communicating more complex, variable, and abstract information. Perhaps flying insects (unable to place a scent trail) that live in complex, 3-D environments (where a simple dance would not suffice to explain the location of food). For such a species, being able to give sequential directions to a food source ("past the tree, around the bend, under the odd-shaped rock, etc.) would be useful.
This "linguistic evolution" could start by developing a pheromone-based "syntax" using certain fixed concepts already built into the instinctive communication system of the species. For example, they might have a scent meaning "danger", a scent meaning "good nesting location" and another scent meaning "food". A dance that incorporated these three scents would then mean, "go towards the place where the predator lives, then turn toward the tree that would make a good nest, and you will find food there".
It is possible that such a language would eventually incorporate sounds as well as scents. Many insects can make noise, though the mouth is only one option for this; others will rub their legs or wings together, vibrate specialized membranes on the sides of their body, or even stomp the ground. There is no reason why an insect species on the verge of developing language couldn't develop a nuanced, sound-based means of communication. For certain forms of information, sounds may be more useful - scents have the problem of "blurring together", so sequential information might be easier to convey through sounds. It probably wouldn't be necessary though, so I wouldn't expect this to develop unless the species already used sound as a part of its "dance".
As this language became more nuanced, the insects might develop more intelligence to store the greater amount of available information. Over time, the language might evolve to convey more abstract concepts - a rock that resembles a typical predator might use the scents or sounds for "danger" and "rock" even though the rock itself is not dangerous; it simply reminds the listener of something dangerous. This could lead to culture, where the insects in a particular nest develop culturally-accepted symbols for specific locations or abstract concepts - also known as "words" - and eventually making their way to true abstract language.