Bhashya
ते किं भवन्तीत्युच्यते— ते कीटाः पतङ्गाः, यदिदं यच्चेदं दन्दशूकं दंशमशकमित्येतत्, भवन्ति ।एवं हि इयं संसारगतिः कष्टा, अस्यां निमग्नस्य पुनरुद्धार एव दुर्लभः ।तथा च श्रुत्यन्तरम्— 'तानीमानि क्षुद्राण्यसकृदावर्तीनि भूतानि भवन्ति जायस्व म्रियस्व' इति ।तस्मात्सर्वोत्साहेन यथाशक्ति स्वाभाविककर्मज्ञानहानेन दक्षिणोत्तरमार्गप्रतिपत्तिसाधनं शास्त्रीयं कर्म ज्ञानं वा अनुतिष्ठेदिति वाक्यार्थः ; तथा चोक्तम्— 'अतो वै खलु दुर्निष्प्रपतरं तस्माज्जुगुप्सेत' इति श्रुत्यन्तरात् मोक्षाय प्रयतेतेत्यर्थः ।अत्रापि उत्तरमार्गप्रतिपत्तिसाधन एव महान् यत्नः कर्तव्य इति गम्यते, 'एवमेवानुपरिवर्तन्ते' इत्युक्तत्वात् ।एवं प्रश्नाः सर्वे निर्णीताः ; 'असौ वै लोकः' इत्यारभ्य 'पुरुषः संभवति' इति चतुर्थः प्रश्नः 'यतिथ्यामाहुत्याम्' इत्यादिः प्राथम्येन ; पञ्चमस्तु द्वितीयत्वेन देवयानस्य वा पथः प्रतिपदं पितृयाणस्य वेति दक्षिणोत्तरमार्गप्रतिपत्तिसाधनकथनेन ; तेनैव च प्रथमोऽपि—अग्नेरारभ्य केचिदर्चिः प्रतिपद्यन्ते केचिद्धूममिति विप्रतिपत्तिः ; पुनरावृत्तिश्च द्वितीय. प्रश्नः— आकाशादिक्रमेणेमं लोकमागच्छन्तीति ; तेनैव— असौ लोको न संपूर्यते कीटपतङ्गादिप्रतिपत्तेश्च केषांचिदिति, तृतीयोऽपि प्रश्नो निर्णीतः ॥
In plain words
So what do they become? A small, crawling, short-lived thing — a worm, a moth, whatever bites, whatever buzzes and stings for a season and dies. This road through birth after birth is genuinely bad news, and once you are this far down in it, climbing back out is close to impossible. Another scripture says exactly this: these small creatures cycle through again and again, told simply, be born, die. So give it everything you have, whatever you're capable of — set aside the merely instinctive living and thinking you were born with, and take up instead the scripturally taught action or knowledge that actually opens the southern path or the northern one. That is what this whole passage is telling you to do. Elsewhere scripture says the same thing more sharply: nothing is harder to break free of than this, so let it fill you with dread — meaning, strive for freedom itself. And notice where the greater effort belongs: toward the northern path especially, since we have already been told plainly that otherwise a person just goes on revolving. With that, every one of the five questions is answered. The passage running from 'that world' to 'a person comes to be' answers the fourth question directly. The fifth is answered along the way, by explaining how one actually enters the path of the gods or the path of the fathers — and that same explanation quietly answers the first question too: some, starting from the fire, reach the flame, others the smoke, which was the very disagreement the first question raised. The second question, about coming back, is answered by tracing the descent, step by step from space downward, into this world. And that same account answers the third: the other world is never overcrowded, because some beings never even arrive there — they fall instead into the bodies of worms and moths.
Close to the Sanskrit
What do they become? — this is explained: they become insects, moths, and whatever else there is of this kind — biting reptiles, gadflies, mosquitoes. For this course of transmigration is truly wretched; for one sunk in it, a way back up is hard indeed to find. So too another scripture says: 'these small creatures, cycling back again and again, are told: be born, die.' Therefore, with all one's effort, according to one's ability, giving up the merely natural action and knowledge one is born with, one should undertake the scripturally enjoined action or knowledge that is the means to attaining the southern or the northern path — this is the meaning of the passage. So too it is said: 'nothing is harder to escape than this; therefore one should recoil from it' — from another scripture; the meaning is that one should strive for liberation. Here too it is understood that great effort in particular should be made toward the means of attaining the northern path, because it has already been said, 'thus indeed they go on revolving.' In this way all the questions have been settled. From 'that world, indeed' up to 'a person comes to be' answers the fourth question, 'at which oblation...' and so on, primarily. The fifth question, however, is answered secondarily, by the exposition of the means of attaining the path of the gods or the path of the fathers. And by that very exposition the first question too — 'beginning from Agni, some attain the flame, some the smoke' — that difference is resolved. The second question, about the return again, is resolved by showing that they come back to this world in due sequence beginning from space. And by that same account, the third question too is resolved: that world is never filled up, because some fall instead into the state of insects, moths, and the like.