DERVISCII. – The word d., from the Persian darwāf, is commonly explained as “beggar” (from dār = “door”), but its etymology is in fact quite uncertain. In most of the Islamic world this word denotes a member of a religious confraternity; in Persian and Turkish it is also used in the narrower sense of mendicant monk.
Religious confraternities began to arise in Islam only in the sixth century of the Hijra, whereas before that time mystical life rested upon an individualistic basis. The first confraternity among those still existing to have a clearly defined historical origin seems to be that of the Qādirites, founded by ‘Abd al-Qādir Gilānī (d. in the year 561 of the Hijra). The origin of the formulas and rites proper to each confraternity is customarily traced, through a long chain (silsilah) of saints, back to the Prophet and to God himself. Every d. is required to know this chain and is also required to believe that the faith professed by his confraternity is the inner essence of Islam. The aspiring dervish (murīd) is “initiated” by his spiritual father (šefh, pir, ustād) by means in which hypnotism sometimes plays a part, and is introduced into the confraternity by means of a contract ('ahd), consisting of a profession of religious faith and vows that vary according to the confraternities. The theological systems of the various confraternities range from ascetic quietism to more or less complete pantheism. The same confraternity may, moreover, assume different forms in different countries: in general, Persian and Turkish confraternities are farther removed from orthodox Islam than are Syrian, Arab, or African ones. The religious rites proper to the confraternity are varied and often produce hypnotic phenomena and ecstatic crises. A rite common to all is the dīhr, whose purpose is to inspire the believer with the thought of the world beyond and of his dependence upon II. The dīhr induces intense religious exaltation, which is further heightened by other means employed by certain orders of mystics (circular dances among the so-called “whirling” d., whose order was founded by the celebrated Persian poet Galāl ad-dīn Rūmī, shouts, etc.).
Some d. perform, for example, extraordinary acts such as swallowing burning coals, live snakes and scorpions, inserting long needles into their bodies and nails into their heads, etc.
Besides the regular members of the orders, who are few and live in monasteries (hangah, ribāt, zāwījah, tekkh) or travel about as mendicant monks, there are adherents comparable to those of our “third orders,” who are required to recite certain daily prayers and occasionally attend a dīhr in the monasteries.

(courtesy of Signora De Ruggiero)
DE RUGGIERO, GUIDO - RITATTO.
The number of Muslim religious confraternities is enormous. One of the best known in Italy is the Cyrenaican confraternity of the Senussi.