Objection 1. It would seem that modesty is not a part of temperance. For modesty is denominated from mode. Now mode is requisite in every virtue: since virtue is directed to good; and "good," according to Augustine (De Nat. Boni 3), "consists in mode, species, and order." Therefore modesty is a general virtue, and consequently should not be reckoned a part of temperance.
Objection 2. Further, temperance would seem to be deserving of praise chiefly on account of its moderation. Now this gives modesty its name. Therefore modesty is the same as temperance, and not one of its parts.
Objection 3. Further, modesty would seem to regard the correction of our neighbor, according to 2 Timothy 2:24-25, "The servant of the Lord must not wrangle, but be mild towards all men... with modesty admonishing them that resist the truth." Now admonishing wrong-doers is an act of justice or of charity, as stated above (Question 33, Article 1). Therefore seemingly modesty is a part of justice rather than of temperance.
On the contrary, Tully (De Invent. Rhet. ii, 54) reckons modesty as a part of temperance.
I answer that, As stated above (141, 4; 157, 3), temperance brings moderation into those things wherein it is most difficult to be moderate, namely the concupiscences of pleasures of touch. Now whenever there is a special virtue about some matter of very great moment, there must needs be another virtue about matters of lesser import: because the life of man requires to be regulated by the virtues with regard to everything: thus it was stated above (134, 3, ad 1), that while magnificence is about great expenditure, there is need in addition for liberality, which is concerned with ordinary expenditure. Hence there is need for a virtue to moderate other lesser matters where moderation is not so difficult. This virtue is called modesty, and is annexed to temperance as its principal.
Reply to Objection 1. When a name is common to many it is sometimes appropriated to those of the lowest rank; thus the common name of angel is appropriated to the lowest order of angels. On the same way, mode which is observed by all virtues in common, is specially appropriated to the virtue which prescribes the mode in the slightest things.
Reply to Objection 2. Some things need tempering on account of their strength, thus we temper strong wine. But moderation is necessary in all things: wherefore temperance is more concerned with strong passions, and modesty about weaker passions.
Reply to Objection 3. Modesty is to be taken there for the general moderation which is necessary in all virtues.
Source: Summa Theologica (New Advent)