where should one draw the line: animals, insects, bacteria?

Rights are to be accorded to creatures that have the capacity to experience pain, to suffer, and to be a "subject of a life".  such a capacity is definitely not found in bacteria.  it is definitely found in mammals.  one should decide, based upon available evidence and one's own conscience, where the line should be drawn to adhere to the principle of animal right described in the capacity of experiencing pain.

do i have to be careful not to walk on ants?

regardless of how careful we are, we will cause some suffering as a side-effect of living.  the goal is to avoid unnecessary suffering and to minimize the suffering we cause. 
this question is an issue for the individual conscience to decide.  as long as, you are not torching a congregation of ants for pleasure.

is it okay to study about animals in zoos?

to gain true and complete knowledge of wild animals, one must observe them in their natural habitats.
the conditions under which animals are kept in zoos typically distorts their behavior.

zoos often mistreat animals, keeping them in small pens or cages.  this is unfair and cruel.
the natural instincts and behavior of these animals are suppressed by force.

Interesting Arguments for Laboratory Animals

What is wrong with experimentation on animals?

Atrocities are not less atrocities when they occur in laboratories and are called medical research.
George Bernard Shaw (Playwright, Nobel 1925)

Vivisection is the blackest of all the black crimes that a man is at present committing against God and his fair creation.
Mahatma Gandhi (statesman and philosopher)

What I think about vivisection is that if people admit that they have the right to take or endanger the life of living beings for the benefit of many, there will be no limit for their cruelty.
Leo Tolstoy (author)

what's wrong with killing a few more rats for medical research?

Farmers have to kill pests to protect our food supply.
Given that, what is wrong with killing a few more rats for medical research?

Humans are killed legitimately every day.  Given that, what is wrong with killing a few more humans for medical research?

How do you know if a product has been tested on animals?

There are 2 ways to determine if a product uses animal products or is tested on animals.
First, most companies provide a toll-free telephone number for inquiring about their product. 

Second, several excellent guides are available that provide listings of companies and products.

But are they reliable?

Animals kill and eat each other; should it be right for humans?

Even thought animals kill and eat each other, humans have a choice: they need not eat meat to survive.

Humans is different from nonhuman animals because humans are capable of conceiving of a system of morals.  Also humans act in accordance with a system of morals.

Don't animals live longer in zoos than they would in the wild?

It sounds like irrelevant.  Suppose a zoo decides to exhibit human beings.  They snatch a peasant from a less-developed country and put her on display.  Due to the regular feedings and health care that the zoo provides, the peasant will live longer in captivity. 
A tradeoff of quantity of life versus quality of life is not always decided in favor of quantity.

長沙灣的鴿子

間中乘巴士經過長沙灣, 在那個東京街的地鐡出口總會有一大堆鴿子. 不用猜,必然是有人在那裡餵鴿子.

天有好生之德, 令沒東西吃的動物有東西吃本來是好事. 反過來說, 不餵牠們, 令牠們沒東西吃而餓死豈不是很殘忍? (又回到那個不捐款給非洲饑民問題

事實上, 越餵鴿子就越多, 衛生就會惡化. 最後大概是衛生部門出手, 結果鴿子星散, 在我們看不到的地方慢慢餓死.

對流浪於人類生活圈的動物,  見即殺有人認為殘忍, 視而不見有人認為冷漠, 餵飼看來只是自我感覺良好. 你認為那種手法比較合理?

街頭餵雀 增擴散疫情危機

食蟲?

在課堂上, 一直在討論為什麼人類可以吃動物而沒有道德問題.

因為比較高等的動物能夠suffer, 所以傷害動物和人都一樣是不好的事.
而比較低等的生物因為它們的知性層面低, 道德上的沖擊相對比較小

所以吃昆蟲在道德的意義上比較合理, 可是一般人都很不喜歡吃昆蟲, 甚至是昆蟲的食品原料

上星期有新聞報導星巴克飲品用蟲屍調色 (http://news.sina.com.hk/news/14/1/1/2622130/1.html)

在修詞上,蟲屍是正確的, 但用同樣手法, 炸雞肶其實也不過是"熱的鳥類屍體".
當你知道那杯東西的原料的時候, 跟你的想法會跟以前一樣嗎?