Theory:

“Sweet creature!” said the Spider, “you’re witty and you’re wise,
How handsome are your gauzy wings, how brilliant are your eyes!
I’ve a little looking-glass upon my parlour shelf,
If you’ll step in one moment, dear, you shall behold yourself.”

“I thank you, gentle sir,” she said, “for what you’re pleased to say,
And bidding you good morning now, I’ll call another day.”

The Spider turned him round about, and went into his den,
For well he knew the silly Fly would soon come back again:
So he wove a subtle web, in a little corner sly,
And set his table ready, to dine upon the Fly.

Then he came out to his door again, and merrily did sing,
“Come hither, hither, pretty Fly, with the pearl and silver wing;
Your robes are green and purple — there’s a crest upon your head;
Your eyes are like the diamond bright, but mine are dull as lead!”

Alas, alas! how very soon this silly little Fly,
Hearing his wily, flattering words, came slowly flitting by;
With buzzing wings she hung aloft, then near and nearer drew,
Thinking only of her brilliant eyes, and green and purple hue —
Thinking only of her crested head — poor foolish thing!

At last,
Up jumped the cunning Spider, and fiercely held her fast.
He dragged her up his winding stair, into his dismal den,
Within his little parlour — but she ne’er came out again!

And now dear little children, who may this story read,
To idle, silly flattering words, I pray you ne’er give heed:
Unto an evil counsellor, close heart and ear and eye,
And take a lesson from this tale, of the Spider and the Fly
Explanation:
  
The spider tried everything to attract the little fly. The spider said the little fly that it was a sweet creature and was bright as well as humorous. The spider praised the wings of the little fly and spoke out that they are very shiny and that the eyes of the small-scale insect were breath-taking. The spider said there is a mirror on the shelf inside the house, and that the little fly can enter the house and see how beautiful it was.
 
The little insect did not fall into all this praise. She just thanked the spider gently and said that she would like to go and wished him a good morning. The little fly spoke out that it will come another day to his house.

The spider went inside his house and waited as he was sure that the little fly would come back again. So, the spider wove a web in a corner to catch the little fly and arranged his table to eat it.
 
The spider returned back to the door and started singing for the little fly. Come flying inside the house with your beautiful wing, which is studded with pearl and looks like silver. Your clothes with purple and green colour and the antennas on your head look like a crown. The eyes are like diamonds, and that his eyes are dull like lead.
 
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The fly, which heard all these, fell in the trap, after hearing the words of the wily spider. With the buzz sound made by the wings, the fly sat near the spider. She went very close to the spider, flattered by his words. The fly was foolish to trust the spider. The spider was very happy on seeing the little fly.
 
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Finally, the spider caught hold of the fly tightly and did not let it go. It dragged the fly through the stairs to its den. The fly never came out again from the spider's clutches. The poet now tells that whom so ever has read this story to understand that never fall prey to these types of flattering words. When someone tries to flatter you by unnecessary words, fairly don't pay attention, keep your heart, eyes, and ears open. That, precisely, is the moral of this poem. The poet gave an example of the spider and the fly to make the children recognize that these wrong people will try to woo you by talking sweetly about you, but they should keep their eye, ears, and heart closed. It is because we see the wrong people who give chocolates or candies to small children to kidnap them or tell them that his/her parents have sent them to pick them up from school. So, don't trust these people blindly. Think, take the help of some adults known to you.
 
Meanings of difficult words:
 
Numbers
Word
Meanings
1.
Gauzy Transparent.
2.
Bidding Instructing.
3.
Flitting Flutter.
4.
Dismal Gloomy.
5.
Crested Crown.
6.
CounsellorAdviser.
Reference:
State Council of Educational Research and Training (2019). Term-2 English Standard-9. The Spider and the Fly- Mary Botham Howitt(pp. 106-107). Published by the Tamil Nadu Textbook and Educational Services Corporation.