According to the Quran or holy scriptures, is it made clear that animals go to heaven or not? As an aside, does the definition include insects?
According to verse no.5 in surat Al-Takwir in Quran (chapter-81) and also chapter-6/verse-38, and many hadiths in Sunnah, animals are questioned at the day of judgement. But there is no evidence that they will enter heaven. In fact, there is at least two hadiths informing explicitly that all animals are turned into earth after being judged hereafter.
I have read in various sites that animals will not go to heaven because they will be turned to dust. But aren't humans also turned to dust? Ashes to ashes, dust to dust. does that necessarily mean that animals wouldn't go to heaven? What about their souls. Maybe the command to turn their bodies to dust also follows that their souls go directly to heaven since they are not actually sinners that need to go to hell. It seems that any transgression they caused is reconciled on judgement day and they could just go straight to heaven. I wish that anyway. When I see the elephants suffering in India, horses in Central Park suffering or dogs being abused or cats murdered for fun by cruel people, etc. How miserable to think that all they got was torment and no joy even in an afterlife. : (
Holy Quran does not overtly deal with the question of animal's place in afterlife. There are two verses which are often quoted in this regard. One states that animals and birds are communities just like people and that we haven't exempted anything in this book. It stops just here. The other one states that everything on the day of judgement will be questioned and every sheep without horns will settle its score with sheep with horn. I personally don't think that the meaning of the verse should be taken literally. Sheep with and without horns is a metaphorical way of differentiating between strong and weak and in no way stand for the class of non-humans.
The question, in earlier history, wouldn't have concerned Islamic scholars and historians at all. And must have only been addressed when Islam's spread began in order to accommodate people of different geographies and cultures. Now the question concerns to fiqh (Islamic jurisprudence) and faqih decides the place of animals in cosmology using his knowledge and reasoning power. Note that faqih of each sect is liable to give different answer in order to demarcate his own sect's position from those of others. This amounts to saying that whenever we will come across some answer to the question of animals place in afterlife, the answer will be partially political rather than a universal solution which could put the question at rest.
But this is the case with every Semitic religion. Religion at its inception couldn't possibly have an answer. They are always developed in the process of its spread. Thus it is only sign of the times that we now see Pope declaring that animals have a place in heaven. Similarly, we find Muslim mufti's overseas issuing fatwas regarding keeping dogs as pets and sea food as halal.
However, if you wish to seek an answer in confinements of sect which theoretically is most tolerant of animals, again, in terms of animals as pets and animals as consumable, then you need to read Imam Malik's texts. Then there's shafi and Jafferi sect which are somewhat tolerant of animals. Holy Prophet's cousin Ali's sermon are rich with animals' instances. He has devoted a whole sermon on peacock. And there are some rather mystical quotes regarding bees and ants.
So in the end, if you reiterate whether it has been made clear in holy scripture which is binding to all sects. The answer is No! It hasn't been made clear. As I said, it falls upon faqih and mujtahid to make it clear and not upon an ordinary Muslim individual (which is sad). If you ever happen to find a clear cut answer, remember, that the answer would be targeting only a particular sect and not Islam in general.
P.s. I am going to assume that you need an answer for research purpose or general knowledge rather than reinforcing your own religious's beliefs, whatever they be, to the effect of accumulating hate for others' beliefs.