Why does Steinbeck refer to the tractors as "insects"? What's his purpose?

His purpose is to convey a negative tone toward the tractors. They are similar to insects that destroy crops, and just like a plague of insects, the tractors are virtually unstoppable.

Explanation:

He think basically that tractors destroy things and are unstoppable just as insects do with plagues

i decided to stay late in the laboratory to continue my experiment with the lightbulb. at midnight, thomas edison opened the door and walked in with his head held high.

1. he leaned on the wall and observed me carefully.

2. his harsh scrutiny made me self-conscious of my skills in the lab.

3. perhaps it was just me, but the air in the laboratory began to thicken as i found myself having difficulty to breathe.

4. i turned to look at him fully as i mentally prepared myself for what my very first words would be to thomas edison.

5. the next couple of seconds shocked me. it wasn't me who spoke, but rather him and with diction i could never think of using in a simple sentence.

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hey, sorry if this was bad, i really didn't know what to write but i wanted to you. is there anything that you had to include within the excerpt?